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Capuli cherry-mediated green synthesis of silver nanoparticles under white solar and blue LED light
Particuology 24 (2016) 123–128
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Particuology journal homepage: www.elsevier.com/locate/partic
Capuli cherry-mediated green synthesis of silver nanoparticles under white solar and blue LED light Brajesh Kumar ∗ , Yolanda Angulo, Kumari Smita, Luis Cumbal, Alexis Debut Centro de Nanociencia y Nanotecnologia, Universidad de las Fuerzas Armadas-ESPE, Av. Gral. Rumi˜ nahui s/n, Sangolqui P.O. BOX 171-5-231B, Ecuador
a r t i c l e
i n f o
Article history: Received 11 March 2015 Received in revised form 8 May 2015 Accepted 11 May 2015 Available online 21 August 2015 Keywords: Silver nanoparticles Capuli LED light Fourier transform infrared spectroscopy Transmission electron microscopy Antioxidant
a b s t r a c t In this article, the Capuli (Prunus serotina Ehrh. var. Capuli) cherry extract was used for the synthesis of silver nanoparticles (AgNPs) in the presence of white/visible solar and blue light-emitting diode (LED) light. For the characterization of the extract and the AgNPs, Fourier transform infrared spectroscopy and ultraviolet–visible spectroscopy were employed, along with hydrodynamic particle size analysis, transmission electron microscopy and X-ray diffraction. The Ag nanospheres obtained using white light were 40–100 nm in diameter and exhibited an absorption peak at max = 445 nm, whereas those obtained using blue LED light were 20–80 nm in diameter with an absorption peak at max = 425 nm. Thermal analysis revealed that the content of biomolecules surrounding the AgNPs was about 55–65%, and it was also found that blue LED light AgNPs (56.28%, 0.05 mM) had a higher antioxidant efficacy than the white solar light AgNPs (33.42%, 0.05 mM) against 1,1-diphenyl-2-picrylhydrazyl. The results indicate that obtaining AgNPs using a blue LED light may prove to be a simple, cost-effective and easily reproducible method for creating future nanopharmaceuticals. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction There has been a continued and considerable interest in nanoparticles for the last few decades owing to the unique properties they display with decreasing size or structural units. Among inorganic-based nanomaterials, silver nanoparticles (AgNPs) are of special interest and are advantageous in the areas of biomolecular detection and diagnostics (Schultz, Smith, Mock, & Schultz, 2000), therapeutics (Eckhardt et al., 2013), catalysis (Kumar, Smita, Cumbal, Debut, & Pathak, 2014), micro-electronics (Gittins, Bethell, Nichols, & Schiffrin, 2000), and many other biomedical and engineering applications. Recently, green synthesis of AgNPs using plant materials has been gaining increased attention because of its ease of preparation, cost effectiveness, and lack of hazardous reagents (Akhtar, Panwar, & Yun, 2013). Plant materials, including leaves (Kumar, Smita, Cumbal, & Debut, 2014a), flowers (Philip, 2010), fruits (Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), seeds (Otari, Patil, Ghosh, & Pawar, 2014), oil (Kumar, Smita, Cumbal, & Debut, 2014b), bark (Sathishkumar et al., 2009), biomass (Kumar, Smita, Cumbal, & Debut, 2014c), and roots (Sreekanth,
∗ Corresponding author. Tel.: +593 2 3989492. E-mail address: [email protected] (B. Kumar).
Ravikumar, & Eom, 2014) have already been reported in the literature. Since ancient times, both primary and secondary metabolites of plants (phytochemicals) have demonstrated their importance in human health applications, but the use of phytochemicals for the synthesis of metal nanoparticles still remains unexplored and is an area of great research potential. The Capuli (Prunus serotina Ehrh. var. Capuli) cherry (Fig. 1(a)) is one of the most common fruits available on the highlands of Ecuador, Peru, Colombia, and Venezuela from December to February. It is similar in shape and taste to the black and Bing cherries. Its thin peel is reddish-purple with a slightly bitter taste, and the pulp is green. It is mostly eaten fresh, stewed, preserved, or made into jam or wine (Dugo, Mondello, Errante, Zappia, & Dugo, 2001). Its major phytochemical components include chlorogenic acid ((+)-catechin and (–)-epicatechin), proanthocyanidin (cyanidin-3-O-glucoside and cyanidin-3O-rutinoside), and flavonol glycosides (quercetin-3-xyloside, quercetin-3-arabinoside) (Vasco, Riihinen, Ruales, & Kamal-Eldin, 2009). Recently, there has been remarkably rapid progress in the development of photocatalytic reactions sensitized by high-brightness light-emitting diodes (LEDs). Several advantages are associated with the use of LED lamps as a light source, such as localized surface plasmon resonance enhancement (Gu, Qiu, Zhang, & Chu,
http://dx.doi.org/10.1016/j.partic.2015.05.005 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Fig. 1. (a) Photograph of Capuli cherries and, (b) the FTIR spectrum of the Capuli cherry extract.
2011), morphology and optical properties control (Stamplecoskie & Scaiano, 2010), improved membrane performance (Li, Verbiest, Strobbeb, & Vankelecom, 2014), high photon efficiency, low voltage of electricity, power stability, accuracy and low cost (Yeh, Wu, & Cheng, 2010). Owing to the high importance of and advances in the application of LEDs in photocatalytic reactions, we study the synthesis of AgNPs using blue LED light. The green synthesis of differently shaped and sized AgNPs using plant materials has already been indicated by our group (Kumar, Smita, Cumbal, Debut, & Pathak, 2014, Kumar et al., 2014a, 2014b, Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), so the main objective of the present study is to compare the synthesis of Ag nanospheres using Capuli cherry extract (CCE) in the presence of white solar and blue LED light. In this work, the CCE acts as a reducing and capping agent, whereas the white solar and blue LED light activates the AgNP synthesis. The CCE and AgNPs are further characterized by Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, hydrodynamic particle size analysis, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The antioxidant efficacy of the AgNPs is also evaluated using 1,1-diphenyl-2-picrylhydrazyl (DPPH• ) to determine whether these nanoparticles can be exploited as an antioxidant carrier source.
Material and methods Synthesis of AgNPs Silver nitrate (AgNO3 , 99.5%) was purchased from Spectrum, USA, the 1,1-diphenyl-2-picrylhydrazyl (DPPH• , >99.5%) was purchased from Sigma Aldrich, USA, and Milli-Q® water was used in all experiments. Fresh Capuli cherries were collected from the open market near Universidad de las Fuerzas Armadas-ESPE, Sangolqui, Ecuador; after thoroughly washed, Capuli cherries (50 g) were chopped and ultrasonicated in 100 mL of water for 10 min. For ultrasonication, ultrasonic transducer (DAIGGER GE 505, 500 W, 20 kHz) was immersed directly into the reaction solution and set to operate at timed pulses of 30 s on and 30 s off with a pulse amplitude of 72% at 25 ◦ C for 10 min (5 min × 2). After ultrasonication, the purple-brown colored CCE was filtered using Whatman No. 1 paper and stored at 4 ◦ C for further use. For green synthesis, 1.0 mL of the CCE was mixed with 10 mL of 1 mM AgNO3 solution, and either kept in white solar light (65–80 cd/m2 ) at 23–28 ◦ C for 96 h, or in blue LED light (12 V, 0.26–0.28 A) at 25–48 ◦ C for 8 h. The reaction mixtures were monitored at different time intervals. A clear suspension of AgNPs was subsequently prepared by centrifuging the reaction mixture containing the AgNPs at 5000 rpm for 20 min at 22–25 ◦ C, and was then used in the instrumental analysis.
DPPH• assay The scavenging activity of the AgNPs at different time intervals was measured using DPPH• as a free radical model and implementing a method adapted from Kumar et al. (2015) and Kumar, Smita, Cumbal, and Debut (2015). An aliquot of CCE, AgNPs or H2 O as control (1.0–0.2 mL) and H2 O (1.0–1.8 mL) was mixed with 2.0 mL of 0.2 mM (DPPH• ) in 95% ethanol. The mixture was vortexed vigorously and allowed to stand at room temperature for 30 min in the dark. Absorbance of the mixture was measured spectrophotometrically at 517 nm, and the free radical scavenging activity was calculated using the relation:
Scavenging effect (%) = 1 −
absorbance of sample × 100 absorbance of control
(1)
The scavenging percentage of all of the samples were plotted, and the final result was expressed as the % of DPPH• free radical scavenging activity (mL). Characterization of AgNPs FTIR attenuated total reflection spectra were recorded on a Spectrum Two IR spectrometer (Perkin Elmer, USA) to detect the different functional groups in the CCE involved in nanoparticle synthesis. The synthesized AgNPs were characterized with a UV–Vis single-beam spectrophotometer (GENESYSTM 8, Thermo Spectronic, UK), while the hydrodynamic size distributions of the nanoparticles were analyzed using a dynamic light scattering (DLS) instrument (LB-550, Horiba, Japan). Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were recorded digitally (Tecnai G2 Spirit TWIN, FEI, Holland). X-ray diffraction (XRD) studies on thin films of the nanoparticle were carried out using a diffractometer (EMPYREAN, PANalytical) and a –2 configuration (generator–detector), wherein a copper X-ray tube emitted a ˚ Thermo gravimetric analysis (TGA) and wavelength of = 1.54 A. differential thermal analysis (DTA) were carried out (TGA Q 500, TA Instruments, USA) by heating 5–10 mg of the sample up to 500 ◦ C in a platinum sample cup at the rate of 20 ◦ C/min. Results and discussion FTIR analysis of CCE FTIR analysis was used to determine the functional groups present in the CCE (Fig. 1(b)). The broad band seen at 3281 cm−1 reveals the presence of an OH group, resulting from either alcoholic or polyphenolic stretching (Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), while the peaks around 2919 and 2850 cm−1
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Fig. 2. UV–Vis spectrum of the AgNPs synthesized under (a) white solar light and (b) blue LED light, including the curve obtained from the CCE.
are attributed to an asymmetric stretching vibration of the C H bond in alkanes (Kumar et al., 2014a). The peaks around 1613 cm−1 may be attributed to C O/amide I groups (Kumar et al., 2014a; Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), and those at 1421 and 1238 cm−1 to C C/C OH groups (Sreekanth et al., 2014). The strong peak observed at 1017 cm−1 corresponds to secondary OH stretching of glucose or carbohydrate (Kumar et al., 2014a) in the CCE. The FTIR results confirm the presence of phytochemicals in the CCE such as chlorogenic acid, proanthocyanidin, and flavonol glycosides (Vasco et al., 2009), which further act as reducing/capping agents for the synthesis of AgNPs.
that is correlated with the localized surface plasmon resonance of spherical AgNPs (Gu et al., 2011; Kumar, Smita, Cumbal, Debut, & Pathak, 2014), while the latter reveals a blue wavelength shift from this characteristic absorption that clearly indicates the existence of smaller nanoparticles in the BL-AgNP sample (Lou, Chen, Lic, & Lin, 2014) than are present in the WL-AgNP sample. In addition to the improved reaction efficiency, the BL-AgNPs were stable for 10 months at room temperature, while the WL-AgNPs were stored for 4 months without any signs of precipitation.
TEM–SAED study UV–Vis study Fig. 2 shows the visible appearance and the UV–Vis spectra of AgNPs synthesized under white solar light (WL-AgNPs) and blue LED light (BL-AgNPs). The reduction of Ag+ ions to Ag occurred slowly by CCE in the presence of white solar light with the reaction mixture presenting a reddish-brown color in 96 h. Under blue LED light, however, the reaction time was reduced, and after only 8 h the reaction mixture was seen to be completely reduced and became a bluish-black color. The UV–Vis spectra of the CCE-synthesized WLAgNPs and BL-AgNPs revealed absorption peaks at max = 445 and 425 nm, respectively. The former is a characteristic absorption peak
Fig. 3 shows TEM images of the WL-AgNPs and BL-AgNPs, whose average sizes are 40–100 and 20–80 nm, respectively. We observe that all synthesized AgNPs are well separated, uniform and possess a spherical shape, and that the size decreases when the source light changes from white to blue LED light. These results clearly indicate that the bioreduction of Ag+ by CCE is enhanced by the blue LED light, producing smaller stabilized particles. The bright circular spot in the SAED pattern (Fig. 3(d), (h)) clearly reveals that the WLand BL-AgNPs are spherical and possess a face-centered cubic (FCC) structure, and that the polycrystallinity of the BL-AgNPs is higher than that of the WL-AgNPs (Durai et al., 2014).
Fig. 3. TEM and SAED images of AgNPs: (a)–(c) TEM images of WL-AgNPs and (d) the corresponding SAED; (e)–(g) TEM images of BL-AgNPs and (h) the corresponding SAED.
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Fig. 4. DLS histograms of (a) WL-AgNPs and (b) BL-AgNPs (blue color indicates histogram of as synthesized AgNPs and green color indicates histogram of standard 10 nm nanoparticles) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
DLS study
Thermal analyses of AgNPs
DLS is a fast and easy technique for particle characterization, especially for measuring the hydrodynamic size distribution of colloidal suspensions. The average particle size found by DLS was 264.6 ± 200.6 and 61.1 ± 39.2 nm for WL-AgNPs and BL-AgNPs, respectively (Fig. 4), which is a much larger value for the WL-AgNPs than that found in the TEM results and confirms the polydispersity of the WL-AgNPs. The disparity in the size results from DLS and TEM arises from the screening of small particles by bigger ones in DLS and also the presence of unreacted phytochemicals of the CCE in the reaction mixture used for DLS (Khlebtsov & Khlebtsov, 2011). The average DLS size of the BL-AgNPs is in good agreement with the TEM image, however, and confirms the monodispersity of the BL-AgNPs (Khlebtsov & Khlebtsov, 2011). In general, the DLS size value is defined for a ball model, which has the same diffusion coefficient as a measured nanoparticle. As a result, the size of measured nanoparticles can differ from that determined by atomic force microscopy and TEM techniques (Tomaszewska et al., 2013).
To investigate the thermolytic behavior of the CCE and the compositions of the Ag NPs, TGA/DTA analyses were performed. Fig. 6 plots the TGA and the first derivative curves (DTA) of the WL-AgNPs and BL-AgNPs prepared with CCE. We notice from the first derivative curves that the weight loss is a two-step desorption for both the WL-AgNPs (crystallization temperature (Tc ) at 116 and 213 ◦ C, Fig. 6(a)) and the BL-AgNPs (Tc at 122 and 210 ◦ C, Fig. 6(b)), and we therefore suggest that the process is exothermic with a loss of H2 O and CO2 (Dong et al., 2009; Han, Yang, Shen, Zhou, & Wang, 2004). The weight losses found for WL-AgNPs and BL-AgNPs are 64.8% and 56.9%, respectively. The discrepancy in the weight losses between the two types of AgNPs is attributed to the different binding sites
XRD study Fig. 5(a) and (b) shows representative XRD patterns from the WL-AgNPs and BL-AgNPs, respectively. The WL-AgNPs exhibit a very weak peak at 2 = 38.06◦ assigned to the (1 1 1) planes of Ag FCC structure. This weakness is caused by the presence of CCE phytochemicals (bio-organic phase) on the particle surface (32.4◦ ) that make it amorphous. Moreover, the BL-AgNPs exhibit strong 2 peaks at 38.06◦ , 44.28◦ , 64.48◦ , and 77.41◦ that can be ascribed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) Bragg reflection planes of the FCC structure of Ag (Umadevi, Bindhu, & Sathe, 2013), which is consistent with the standard data file ICSD No. 98-004-4387. The high intensity peak at 38.06◦ suggests that the (1 1 1) plane is the predominant orientation and indicates a high degree of crystallinity (Kumar et al., 2014a). The diffraction peaks are broad, however, and Debye–Scherrer equation predicts that the particle size is less than 70 nm for the BL-AgNPs.
Fig. 5. XRD patterns of (a) WL-AgNPs and (b) BL-AgNPs.
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where it can be seen that the scavenging activity of CCE increases with increasing concentration, with the maximum value (67.04%) found for a quantity of 1 mL. However, the antioxidant activity of the as-prepared AgNPs differs. The maximum scavenging percentage for the WL-AgNPs measured as 52.36% in 0.6 mL/0.15 mM, whereas it is observed to be 56.25% in 0.2 mL/0.05 mM for BLAgNPs. This clearly indicates that the BL-AgNPs possess a higher antioxidant activity, with an antioxidant ranking of BL-AgNPs » WLAgNPs > CCE at lower concentrations. This increased antioxidant activity is owing to the involvement of the CCE phytochemicals on the surface of the stabilized AgNPs (Kumar, Smita, Cumbal, & Debut, 2014d; Sreekanth et al., 2014), and also to the stabilization of the AgNPs activated by blue LED light irradiation. Also, the surface-to-volume ratio is higher for smaller nanoparticles, and this increased surface exposure may more easily scavenge the free radicals and create an enhancement of antioxidant efficacy (Singh, Amateis, Mahaney, Meehan, & Rzigalinski, 2008; Tamjid, Bagheri, Vossoughi, & Simchi, 2011). Conclusions
Fig. 6. TGA and the first derivative curves of (a) WL-AgNPs and (b) BL-AgNPs.
of CCE on the AgNPs surfaces. Furthermore, the amorphous nature of the WL-AgNPs in the SAED and XRD measurements is supported by the results of the thermal analysis measurements due to the presence of thicker coated CCE on the surface of WL-AgNPs. Antioxidant efficacy The antioxidant efficacy of the CCE, WL-AgNPs, and BL-AgNPs was investigated using a DPPH• assay with ethanol (95%), wherein the DPPH• color in the ethanol changed from purple to yellow in the presence of antioxidant molecules. The results are shown in Fig. 7,
In conclusion, the use of CCE in the presence of blue LED light promotes the reduction of Ag+ to Ag nanospheres while avoiding the use of toxic or hazardous chemicals. The UV–Vis analysis shows the absorption peak, max , for AgNPs synthesized using blue LED light is blue-shifted to 425 nm, whereas it is 445 nm for those synthesized with white light. The TEM and DLS measurements show that the prepared BL-AgNPs are spherical and monodispersed with an average size of 61.1 ± 39.2 nm. The SAED and XRD pattern confirms the crystalline nature of the AgNPs with FCC symmetry. Enhanced antioxidant efficacy of the BL-AgNPs (56.28%, 0.05 mM) and WL-AgNPs (33.42%, 0.05 mM) are observed, owing to the encapsulation of CCE phytochemicals on the surface of the particles. The present study shows, therefore, that this process may be useful for the sustainable synthesis of nanoparticles and for their applicability in different biotechnology fields. Acknowledgements This work has been funded by the Prometeo Project of the National Secretariat of Higher Education, Science, Technology and Innovation (SENESCYT), Ecuador. We thank Prof. Victor H. Guerrero (Escuela Politécnica Nacional, Ecuador) for providing the TGA/DTA instrumental facility. References
Fig. 7. The DPPH assay of (a) CCE, (b) WL-AgNPs and (c) BL-AgNPs.
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Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Capuli cherry-mediated green synthesis of silver nanoparticles under white solar and blue LED light Brajesh Kumar ∗ , Yolanda Angulo, Kumari Smita, Luis Cumbal, Alexis Debut Centro de Nanociencia y Nanotecnologia, Universidad de las Fuerzas Armadas-ESPE, Av. Gral. Rumi˜ nahui s/n, Sangolqui P.O. BOX 171-5-231B, Ecuador
a r t i c l e
i n f o
Article history: Received 11 March 2015 Received in revised form 8 May 2015 Accepted 11 May 2015 Available online 21 August 2015 Keywords: Silver nanoparticles Capuli LED light Fourier transform infrared spectroscopy Transmission electron microscopy Antioxidant
a b s t r a c t In this article, the Capuli (Prunus serotina Ehrh. var. Capuli) cherry extract was used for the synthesis of silver nanoparticles (AgNPs) in the presence of white/visible solar and blue light-emitting diode (LED) light. For the characterization of the extract and the AgNPs, Fourier transform infrared spectroscopy and ultraviolet–visible spectroscopy were employed, along with hydrodynamic particle size analysis, transmission electron microscopy and X-ray diffraction. The Ag nanospheres obtained using white light were 40–100 nm in diameter and exhibited an absorption peak at max = 445 nm, whereas those obtained using blue LED light were 20–80 nm in diameter with an absorption peak at max = 425 nm. Thermal analysis revealed that the content of biomolecules surrounding the AgNPs was about 55–65%, and it was also found that blue LED light AgNPs (56.28%, 0.05 mM) had a higher antioxidant efficacy than the white solar light AgNPs (33.42%, 0.05 mM) against 1,1-diphenyl-2-picrylhydrazyl. The results indicate that obtaining AgNPs using a blue LED light may prove to be a simple, cost-effective and easily reproducible method for creating future nanopharmaceuticals. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction There has been a continued and considerable interest in nanoparticles for the last few decades owing to the unique properties they display with decreasing size or structural units. Among inorganic-based nanomaterials, silver nanoparticles (AgNPs) are of special interest and are advantageous in the areas of biomolecular detection and diagnostics (Schultz, Smith, Mock, & Schultz, 2000), therapeutics (Eckhardt et al., 2013), catalysis (Kumar, Smita, Cumbal, Debut, & Pathak, 2014), micro-electronics (Gittins, Bethell, Nichols, & Schiffrin, 2000), and many other biomedical and engineering applications. Recently, green synthesis of AgNPs using plant materials has been gaining increased attention because of its ease of preparation, cost effectiveness, and lack of hazardous reagents (Akhtar, Panwar, & Yun, 2013). Plant materials, including leaves (Kumar, Smita, Cumbal, & Debut, 2014a), flowers (Philip, 2010), fruits (Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), seeds (Otari, Patil, Ghosh, & Pawar, 2014), oil (Kumar, Smita, Cumbal, & Debut, 2014b), bark (Sathishkumar et al., 2009), biomass (Kumar, Smita, Cumbal, & Debut, 2014c), and roots (Sreekanth,
∗ Corresponding author. Tel.: +593 2 3989492. E-mail address: [email protected] (B. Kumar).
Ravikumar, & Eom, 2014) have already been reported in the literature. Since ancient times, both primary and secondary metabolites of plants (phytochemicals) have demonstrated their importance in human health applications, but the use of phytochemicals for the synthesis of metal nanoparticles still remains unexplored and is an area of great research potential. The Capuli (Prunus serotina Ehrh. var. Capuli) cherry (Fig. 1(a)) is one of the most common fruits available on the highlands of Ecuador, Peru, Colombia, and Venezuela from December to February. It is similar in shape and taste to the black and Bing cherries. Its thin peel is reddish-purple with a slightly bitter taste, and the pulp is green. It is mostly eaten fresh, stewed, preserved, or made into jam or wine (Dugo, Mondello, Errante, Zappia, & Dugo, 2001). Its major phytochemical components include chlorogenic acid ((+)-catechin and (–)-epicatechin), proanthocyanidin (cyanidin-3-O-glucoside and cyanidin-3O-rutinoside), and flavonol glycosides (quercetin-3-xyloside, quercetin-3-arabinoside) (Vasco, Riihinen, Ruales, & Kamal-Eldin, 2009). Recently, there has been remarkably rapid progress in the development of photocatalytic reactions sensitized by high-brightness light-emitting diodes (LEDs). Several advantages are associated with the use of LED lamps as a light source, such as localized surface plasmon resonance enhancement (Gu, Qiu, Zhang, & Chu,
http://dx.doi.org/10.1016/j.partic.2015.05.005 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Fig. 1. (a) Photograph of Capuli cherries and, (b) the FTIR spectrum of the Capuli cherry extract.
2011), morphology and optical properties control (Stamplecoskie & Scaiano, 2010), improved membrane performance (Li, Verbiest, Strobbeb, & Vankelecom, 2014), high photon efficiency, low voltage of electricity, power stability, accuracy and low cost (Yeh, Wu, & Cheng, 2010). Owing to the high importance of and advances in the application of LEDs in photocatalytic reactions, we study the synthesis of AgNPs using blue LED light. The green synthesis of differently shaped and sized AgNPs using plant materials has already been indicated by our group (Kumar, Smita, Cumbal, Debut, & Pathak, 2014, Kumar et al., 2014a, 2014b, Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), so the main objective of the present study is to compare the synthesis of Ag nanospheres using Capuli cherry extract (CCE) in the presence of white solar and blue LED light. In this work, the CCE acts as a reducing and capping agent, whereas the white solar and blue LED light activates the AgNP synthesis. The CCE and AgNPs are further characterized by Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, hydrodynamic particle size analysis, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The antioxidant efficacy of the AgNPs is also evaluated using 1,1-diphenyl-2-picrylhydrazyl (DPPH• ) to determine whether these nanoparticles can be exploited as an antioxidant carrier source.
Material and methods Synthesis of AgNPs Silver nitrate (AgNO3 , 99.5%) was purchased from Spectrum, USA, the 1,1-diphenyl-2-picrylhydrazyl (DPPH• , >99.5%) was purchased from Sigma Aldrich, USA, and Milli-Q® water was used in all experiments. Fresh Capuli cherries were collected from the open market near Universidad de las Fuerzas Armadas-ESPE, Sangolqui, Ecuador; after thoroughly washed, Capuli cherries (50 g) were chopped and ultrasonicated in 100 mL of water for 10 min. For ultrasonication, ultrasonic transducer (DAIGGER GE 505, 500 W, 20 kHz) was immersed directly into the reaction solution and set to operate at timed pulses of 30 s on and 30 s off with a pulse amplitude of 72% at 25 ◦ C for 10 min (5 min × 2). After ultrasonication, the purple-brown colored CCE was filtered using Whatman No. 1 paper and stored at 4 ◦ C for further use. For green synthesis, 1.0 mL of the CCE was mixed with 10 mL of 1 mM AgNO3 solution, and either kept in white solar light (65–80 cd/m2 ) at 23–28 ◦ C for 96 h, or in blue LED light (12 V, 0.26–0.28 A) at 25–48 ◦ C for 8 h. The reaction mixtures were monitored at different time intervals. A clear suspension of AgNPs was subsequently prepared by centrifuging the reaction mixture containing the AgNPs at 5000 rpm for 20 min at 22–25 ◦ C, and was then used in the instrumental analysis.
DPPH• assay The scavenging activity of the AgNPs at different time intervals was measured using DPPH• as a free radical model and implementing a method adapted from Kumar et al. (2015) and Kumar, Smita, Cumbal, and Debut (2015). An aliquot of CCE, AgNPs or H2 O as control (1.0–0.2 mL) and H2 O (1.0–1.8 mL) was mixed with 2.0 mL of 0.2 mM (DPPH• ) in 95% ethanol. The mixture was vortexed vigorously and allowed to stand at room temperature for 30 min in the dark. Absorbance of the mixture was measured spectrophotometrically at 517 nm, and the free radical scavenging activity was calculated using the relation:
Scavenging effect (%) = 1 −
absorbance of sample × 100 absorbance of control
(1)
The scavenging percentage of all of the samples were plotted, and the final result was expressed as the % of DPPH• free radical scavenging activity (mL). Characterization of AgNPs FTIR attenuated total reflection spectra were recorded on a Spectrum Two IR spectrometer (Perkin Elmer, USA) to detect the different functional groups in the CCE involved in nanoparticle synthesis. The synthesized AgNPs were characterized with a UV–Vis single-beam spectrophotometer (GENESYSTM 8, Thermo Spectronic, UK), while the hydrodynamic size distributions of the nanoparticles were analyzed using a dynamic light scattering (DLS) instrument (LB-550, Horiba, Japan). Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were recorded digitally (Tecnai G2 Spirit TWIN, FEI, Holland). X-ray diffraction (XRD) studies on thin films of the nanoparticle were carried out using a diffractometer (EMPYREAN, PANalytical) and a –2 configuration (generator–detector), wherein a copper X-ray tube emitted a ˚ Thermo gravimetric analysis (TGA) and wavelength of = 1.54 A. differential thermal analysis (DTA) were carried out (TGA Q 500, TA Instruments, USA) by heating 5–10 mg of the sample up to 500 ◦ C in a platinum sample cup at the rate of 20 ◦ C/min. Results and discussion FTIR analysis of CCE FTIR analysis was used to determine the functional groups present in the CCE (Fig. 1(b)). The broad band seen at 3281 cm−1 reveals the presence of an OH group, resulting from either alcoholic or polyphenolic stretching (Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), while the peaks around 2919 and 2850 cm−1
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Fig. 2. UV–Vis spectrum of the AgNPs synthesized under (a) white solar light and (b) blue LED light, including the curve obtained from the CCE.
are attributed to an asymmetric stretching vibration of the C H bond in alkanes (Kumar et al., 2014a). The peaks around 1613 cm−1 may be attributed to C O/amide I groups (Kumar et al., 2014a; Kumar, Smita, Cumbal, Debut, Camacho, et al., 2015), and those at 1421 and 1238 cm−1 to C C/C OH groups (Sreekanth et al., 2014). The strong peak observed at 1017 cm−1 corresponds to secondary OH stretching of glucose or carbohydrate (Kumar et al., 2014a) in the CCE. The FTIR results confirm the presence of phytochemicals in the CCE such as chlorogenic acid, proanthocyanidin, and flavonol glycosides (Vasco et al., 2009), which further act as reducing/capping agents for the synthesis of AgNPs.
that is correlated with the localized surface plasmon resonance of spherical AgNPs (Gu et al., 2011; Kumar, Smita, Cumbal, Debut, & Pathak, 2014), while the latter reveals a blue wavelength shift from this characteristic absorption that clearly indicates the existence of smaller nanoparticles in the BL-AgNP sample (Lou, Chen, Lic, & Lin, 2014) than are present in the WL-AgNP sample. In addition to the improved reaction efficiency, the BL-AgNPs were stable for 10 months at room temperature, while the WL-AgNPs were stored for 4 months without any signs of precipitation.
TEM–SAED study UV–Vis study Fig. 2 shows the visible appearance and the UV–Vis spectra of AgNPs synthesized under white solar light (WL-AgNPs) and blue LED light (BL-AgNPs). The reduction of Ag+ ions to Ag occurred slowly by CCE in the presence of white solar light with the reaction mixture presenting a reddish-brown color in 96 h. Under blue LED light, however, the reaction time was reduced, and after only 8 h the reaction mixture was seen to be completely reduced and became a bluish-black color. The UV–Vis spectra of the CCE-synthesized WLAgNPs and BL-AgNPs revealed absorption peaks at max = 445 and 425 nm, respectively. The former is a characteristic absorption peak
Fig. 3 shows TEM images of the WL-AgNPs and BL-AgNPs, whose average sizes are 40–100 and 20–80 nm, respectively. We observe that all synthesized AgNPs are well separated, uniform and possess a spherical shape, and that the size decreases when the source light changes from white to blue LED light. These results clearly indicate that the bioreduction of Ag+ by CCE is enhanced by the blue LED light, producing smaller stabilized particles. The bright circular spot in the SAED pattern (Fig. 3(d), (h)) clearly reveals that the WLand BL-AgNPs are spherical and possess a face-centered cubic (FCC) structure, and that the polycrystallinity of the BL-AgNPs is higher than that of the WL-AgNPs (Durai et al., 2014).
Fig. 3. TEM and SAED images of AgNPs: (a)–(c) TEM images of WL-AgNPs and (d) the corresponding SAED; (e)–(g) TEM images of BL-AgNPs and (h) the corresponding SAED.
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Fig. 4. DLS histograms of (a) WL-AgNPs and (b) BL-AgNPs (blue color indicates histogram of as synthesized AgNPs and green color indicates histogram of standard 10 nm nanoparticles) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
DLS study
Thermal analyses of AgNPs
DLS is a fast and easy technique for particle characterization, especially for measuring the hydrodynamic size distribution of colloidal suspensions. The average particle size found by DLS was 264.6 ± 200.6 and 61.1 ± 39.2 nm for WL-AgNPs and BL-AgNPs, respectively (Fig. 4), which is a much larger value for the WL-AgNPs than that found in the TEM results and confirms the polydispersity of the WL-AgNPs. The disparity in the size results from DLS and TEM arises from the screening of small particles by bigger ones in DLS and also the presence of unreacted phytochemicals of the CCE in the reaction mixture used for DLS (Khlebtsov & Khlebtsov, 2011). The average DLS size of the BL-AgNPs is in good agreement with the TEM image, however, and confirms the monodispersity of the BL-AgNPs (Khlebtsov & Khlebtsov, 2011). In general, the DLS size value is defined for a ball model, which has the same diffusion coefficient as a measured nanoparticle. As a result, the size of measured nanoparticles can differ from that determined by atomic force microscopy and TEM techniques (Tomaszewska et al., 2013).
To investigate the thermolytic behavior of the CCE and the compositions of the Ag NPs, TGA/DTA analyses were performed. Fig. 6 plots the TGA and the first derivative curves (DTA) of the WL-AgNPs and BL-AgNPs prepared with CCE. We notice from the first derivative curves that the weight loss is a two-step desorption for both the WL-AgNPs (crystallization temperature (Tc ) at 116 and 213 ◦ C, Fig. 6(a)) and the BL-AgNPs (Tc at 122 and 210 ◦ C, Fig. 6(b)), and we therefore suggest that the process is exothermic with a loss of H2 O and CO2 (Dong et al., 2009; Han, Yang, Shen, Zhou, & Wang, 2004). The weight losses found for WL-AgNPs and BL-AgNPs are 64.8% and 56.9%, respectively. The discrepancy in the weight losses between the two types of AgNPs is attributed to the different binding sites
XRD study Fig. 5(a) and (b) shows representative XRD patterns from the WL-AgNPs and BL-AgNPs, respectively. The WL-AgNPs exhibit a very weak peak at 2 = 38.06◦ assigned to the (1 1 1) planes of Ag FCC structure. This weakness is caused by the presence of CCE phytochemicals (bio-organic phase) on the particle surface (32.4◦ ) that make it amorphous. Moreover, the BL-AgNPs exhibit strong 2 peaks at 38.06◦ , 44.28◦ , 64.48◦ , and 77.41◦ that can be ascribed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) Bragg reflection planes of the FCC structure of Ag (Umadevi, Bindhu, & Sathe, 2013), which is consistent with the standard data file ICSD No. 98-004-4387. The high intensity peak at 38.06◦ suggests that the (1 1 1) plane is the predominant orientation and indicates a high degree of crystallinity (Kumar et al., 2014a). The diffraction peaks are broad, however, and Debye–Scherrer equation predicts that the particle size is less than 70 nm for the BL-AgNPs.
Fig. 5. XRD patterns of (a) WL-AgNPs and (b) BL-AgNPs.
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where it can be seen that the scavenging activity of CCE increases with increasing concentration, with the maximum value (67.04%) found for a quantity of 1 mL. However, the antioxidant activity of the as-prepared AgNPs differs. The maximum scavenging percentage for the WL-AgNPs measured as 52.36% in 0.6 mL/0.15 mM, whereas it is observed to be 56.25% in 0.2 mL/0.05 mM for BLAgNPs. This clearly indicates that the BL-AgNPs possess a higher antioxidant activity, with an antioxidant ranking of BL-AgNPs » WLAgNPs > CCE at lower concentrations. This increased antioxidant activity is owing to the involvement of the CCE phytochemicals on the surface of the stabilized AgNPs (Kumar, Smita, Cumbal, & Debut, 2014d; Sreekanth et al., 2014), and also to the stabilization of the AgNPs activated by blue LED light irradiation. Also, the surface-to-volume ratio is higher for smaller nanoparticles, and this increased surface exposure may more easily scavenge the free radicals and create an enhancement of antioxidant efficacy (Singh, Amateis, Mahaney, Meehan, & Rzigalinski, 2008; Tamjid, Bagheri, Vossoughi, & Simchi, 2011). Conclusions
Fig. 6. TGA and the first derivative curves of (a) WL-AgNPs and (b) BL-AgNPs.
of CCE on the AgNPs surfaces. Furthermore, the amorphous nature of the WL-AgNPs in the SAED and XRD measurements is supported by the results of the thermal analysis measurements due to the presence of thicker coated CCE on the surface of WL-AgNPs. Antioxidant efficacy The antioxidant efficacy of the CCE, WL-AgNPs, and BL-AgNPs was investigated using a DPPH• assay with ethanol (95%), wherein the DPPH• color in the ethanol changed from purple to yellow in the presence of antioxidant molecules. The results are shown in Fig. 7,
In conclusion, the use of CCE in the presence of blue LED light promotes the reduction of Ag+ to Ag nanospheres while avoiding the use of toxic or hazardous chemicals. The UV–Vis analysis shows the absorption peak, max , for AgNPs synthesized using blue LED light is blue-shifted to 425 nm, whereas it is 445 nm for those synthesized with white light. The TEM and DLS measurements show that the prepared BL-AgNPs are spherical and monodispersed with an average size of 61.1 ± 39.2 nm. The SAED and XRD pattern confirms the crystalline nature of the AgNPs with FCC symmetry. Enhanced antioxidant efficacy of the BL-AgNPs (56.28%, 0.05 mM) and WL-AgNPs (33.42%, 0.05 mM) are observed, owing to the encapsulation of CCE phytochemicals on the surface of the particles. The present study shows, therefore, that this process may be useful for the sustainable synthesis of nanoparticles and for their applicability in different biotechnology fields. Acknowledgements This work has been funded by the Prometeo Project of the National Secretariat of Higher Education, Science, Technology and Innovation (SENESCYT), Ecuador. We thank Prof. Victor H. Guerrero (Escuela Politécnica Nacional, Ecuador) for providing the TGA/DTA instrumental facility. References
Fig. 7. The DPPH assay of (a) CCE, (b) WL-AgNPs and (c) BL-AgNPs.
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