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Popcorn-shaped gold nanoparticles: Plant extract-mediated synthesis, characterization and multiphoton-excited luminescence properties
Accepted Manuscript Popcorn-shaped gold nanoparticles: plant extract-mediated synthesis, characterization and multiphoton-excited luminescence properties.
Magdalena Klekotko, Katarzyna Brach, Joanna Olesiak-Banska, Marek Samoc, Katarzyna Matczyszyn PII:
S0254-0584(19)30161-0
DOI:
10.1016/j.matchemphys.2019.02.066
Reference:
MAC 21409
To appear in:
Materials Chemistry and Physics
Received Date:
25 June 2018
Accepted Date:
17 February 2019
Please cite this article as: Magdalena Klekotko, Katarzyna Brach, Joanna Olesiak-Banska, Marek Samoc, Katarzyna Matczyszyn, Popcorn-shaped gold nanoparticles: plant extract-mediated synthesis, characterization and multiphoton-excited luminescence properties., Materials Chemistry and Physics (2019), doi: 10.1016/j.matchemphys.2019.02.066
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ACCEPTED MANUSCRIPT
Popcorn-shaped gold nanoparticles: plant extract-mediated synthesis, characterization and multiphoton-excited luminescence properties. Magdalena Klekotko, Katarzyna Brach, Joanna Olesiak-Banska*, Marek Samoc, Katarzyna Matczyszyn Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wrocław, Poland E-mail: [email protected]
Highlights
Gold nanoparticles were synthesized using the aqueous extract of Cistus incanus. The biosynthesis led to popcorn-shaped gold nanoparticles with highly folded surface. The nanoparticles exhibited multiphoton-excited luminescence. Popcorn-shaped gold nanoparticles may be useful multiphoton imaging agents.
Keywords Gold nanoparticles, Biosynthesis, Multiphoton-excited luminescence properties
Abstract We describe the application of Cistus incanus extract as a source of reducing, structuredirecting and stabilizing agents in the fabrication of gold nanoparticles (GNPs) from tetrachloroauric acid solution. The synthesis resulted in the formation of nanostructures with highly extended surface area, resembling the shape of popcorn. Various ratios of plant extract volume to chloroauric acid volume were investigated in order to obtain nanoparticles with desired shapes having the narrowest size distribution. The synthesized nanoparticles were characterized using UV-Vis absorption spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy (AFM). Multiphoton-excited luminescence (MPL) properties of plant extract synthesized popcorn-shaped gold nanoparticles were examined in comparison with gold nanostars synthesized chemically. The obtained results suggest that the GNPs synthesized using cistus extract may be useful imaging agents for multiphoton microscopy.
ACCEPTED MANUSCRIPT 1. Introduction. Gold nanoparticles (GNPs) offer a wide range of useful properties (such as optical effects related to the tunable surface plasmon resonance (SPR),[1-3] catalytic activity[4-6] or light to heat conversion[7-9]), which are strongly dependent on many factors, including size[10-12] and shape[13-15] of the nanoparticles, coating of the surface[16-18] or their surroundings.[1921] The utility of these structures has been intensively explored revealing numerous applications (e.g. microscopic imaging,[22] targeted therapies,[23] biosensing[24]). Anisotropic, branched nanostructures, such as nanoflowers,[25] nanostars,[1] urchin-like nanoparticles[26] or nanodendrites[27] seem to be particularly interesting due to their highly extended surface area, numerous adsorption sites and significant enhancement of the local electromagnetic field at the branches tips, which may bring many advantages in catalysis,[28] drug delivery systems,[29] imaging[2] or surface-enhanced Raman spectroscopy (SERS).[30] This variety of the GNPs applications has contributed to the great progress in the development of numerous methods of their fabrication.[31-33] Recently, there has been growing interest in “green chemistry” approaches of the GNPs synthesis, employing bacteria[34-36], fungi[37-39] or plants[40-43]. There are multiple examples of successful application of biogenic gold nanoparticles in nanomedicine: as anticancer agents [40, 44], inflammation inhibitors[45] or antimicrobial agents[42, 43, 46]. The advantage of “green chemistry” methods in gold nanoparticle synthesis is the production of the nanostructures in eco-friendly manner without using any toxic chemicals. The stabilizing ligands of nanoparticles are usually formed from alkanoids, flavonoids, alkaloids, proteins or starch[41-43, 47]. However, most of those methods lead to the formation of nanospheres or polydisperse mixtures containing spherical, triangular and hexagonal shapes of the nanoparticles.[48] In this paper we demonstrate for the first time the use of cistus (Cistus incanus) extract for the synthesis of popcorn-shaped gold nanoparticles, characterization of the obtained structures and their multiphoton properties in comparison with star-shaped gold nanoparticles synthesized chemically. It needs to be mentioned that we have carried out many tests of the use of various plant extracts for the fabrication of gold nanoparticles and we found this particular extract to provide results that were the most interesting, because of the particular peculiar shapes of the nanoparticles produced. Cistus incanus is a herbal plant with antioxidant, anti-inflammatory and antimicrobial properties. It is mostly used in the form of infusions or dietary supplements containing its components.[49] In our work aqueous extract of Cistus incanus was used for the synthesis of popcorn-shaped GNPs. It is a simple way to produce nanoparticles without any complex protocols, toxic reagents, harmful wastes or expensive devices. Phytochemicals extracted from the plant act as reducing, structure-directing and stabilizing agents in the process of conversion of gold ions from tetrachloroauric acid solution into gold nanoparticles, however, the exact mechanism of the synthesis is still unknown. There are some assumptions, that biomolecules such as proteins, phenols and flavonoids play a key role in such a system.[48] The obtained nanoparticles exhibit multiphoton luminescence (MPL) properties, which make them potentially useful as imaging probes for multiphoton microscopy, taking advantage of the range of excitation wavelengths placed within the optical therapeutic window, where the light reaches maximal penetration depth into tissues.[50]
ACCEPTED MANUSCRIPT 2. Materials and methods. 2.1. Reagents. Tetrachloroauric(III) acid (HAuCl4•3H2O), sodium citrate tribasic dihydrate (Na3C6H5O7•2H2O), L-ascorbic acid, silver nitrate, 2-mercaptoethanol were obtained from Sigma-Aldrich, hydrochloric acid was obtained from POCH S.A. All reagents were used as received. Dried, powdered leaves of Cistus incanus were purchased in a local market. 2.2. Synthesis of gold nanoparticles. Two types of gold nanoparticles were studied: the popcorn-like ones obtained by using the cistus plant extract and the gold nanostars obtained using a more conventional protocol using typical chemicals. In both cases the gold precursor was tetrachloroauric acid, which was treated by reducing agents in order to convert it to metallic gold. It is well known that the products of such reduction can be quite diverse, depending on many details of the protocol being used. Popcorn-shaped gold nanoparticles were synthesized using water extract of cistus (Cistus incanus). The extract was prepared using 3 g of dried, powdered leaves mixed with 30 mL of distilled, deionized water. The extraction was carried out in a water bath at 100°C for 30 min. Then, the mixture was centrifuged at 5000 rpm (MPW-380 Centrifuge) for 10 min, the supernatant was filtered and stored at 4°C for further experiments. The synthesis of gold nanoparticles was performed by addition of various volumes of the extract (100 L, 200 L, 500 L, 700 L and 1 mL) to 2 mL of 1 mM HAuCl4 aqueous solution. Each of the reaction mixtures was adjusted to the final volume of 3 mL by addition of required amount of deionised water. The obtained solutions were vigorously shaken for 30 s and then left undisturbed for 24 h in the dark, at room temperature. Finally, the synthesized GNPs were centrifuged at 5000 rpm (Eppendorf Centrifuge 5418) for 5 min, supernatant was discarded and the pellet was resuspended in distilled, deionized water. Synthesis of star-shaped gold nanoparticles was based on the two-step protocol of Yuan et al.[14]. In the first step, citrate-stabilized spherical gold nanoparticles (so-called seed solution) were prepared applying the Turkevich method [51]: under vigorous stirring, 5 mL of 1% sodium citrate solution was added to 50 mL of boiling 0.5 mM HAuCl4 aqueous solution. The solution became colourless and after 15 min, ruby-red colour appeared. The solution was cooled down and used as a seed solution in the next step. The growth solution was prepared by mixing 20 mL of distilled, deionized water with 200 L of 25 mM HAuCl4 aqueous solution, 1 mL of the seed solution and 20 L of 1 M HCl solution. The star shape nanoparticles were obtained by simultaneous addition of 100 L of 100 mM ascorbic acid solution and 20 L of 25 mM silver nitrate solution. Rapid change of the mixture colour from light red to dark blue indicated the formation of branched structures. The reaction was stopped after 30 s by addition of 5 L of 2mercaptoethanol. Finally, the obtained gold nanostars were purified by centrifugation at 5000 rpm (MPW-380 Centrifuge) for 5 min and resuspension in distilled, deionized water. 2.3. Characterization of obtained nanoparticles.
ACCEPTED MANUSCRIPT The extinction spectra of the synthesized gold nanoparticles were obtained using a JASCO V670 Spectrophotometer. The measurements were performed in 10 mm quartz cuvettes in the wavelength range between 400 nm and 1200 nm. The morphological features of the GNPs were revealed using transmission electron microscopy (TEM) applying a FEI Tecnai G2 20 X-TWIN microscope. TEM sample preparation involved putting a drop of the solution of the GNPs onto carbon coated copper grids and drying at room temperature. 2.4. Multiphoton-excited luminescence properties. Samples for the multiphoton-excited luminescence measurements were prepared by spreading of 50 L of the nanoparticles solution onto cover glass. Firstly, proper concentrations of the nanoparticles were established by preparation of the samples using a series of the diluted GNPs solutions (10, 100 and 1000 times diluted) and measuring the distribution of the nanoparticles on the surface of the glass applying an atomic force microscope (AFM) (Dimensional V scanning probe microscope, Veeco) operating in the tapping mode. Multiphoton-excited luminescence properties of the gold nanoparticles were examined using a custom-made setup. The samples were placed on the XYZ piezoelectric scanning stage (TRITOR 102) and excited using a mode-locked Ti:sapphire laser (Chameleon, Coherent Inc.), with the excitation wavelength set to 800 nm. A high numerical aperture Nikon Plan Apo oil immersion objective (100×/1.4 NA), working in epifluorescence mode, was used for the focusing of an incident laser beam on the sample and for the collecting of the multiphoton-excited emission. The emitted signal was separated from the incident light on a dichroic mirror (cutoff wavelength 720 nm) and collected by avalanche photodiodes operating in the photon counting regime. 3. Results and discussion. 3.1. Synthesis and characterization of gold nanoparticles. Addition of the cistus extract to 1 mM tetrachloroauric acid aqueous solution leads to a rapid change of the reaction mixture colour from light yellow to a range of colours from purple to dark blue, depending on the extract volume (Fig. 1a). The extinction spectra recorded after the synthesis revealed characteristic surface plasmon resonance bands of the obtained gold nanoparticles (Fig. 1b). The morphology of the synthesized GNPs was determined under transmission electron microscope (Fig. 1c). The first reaction mixture (100 L of the cistus extract per 2 mL of 1 mM HAuCl4 solution) contained mostly spherical nanoparticles with the size up to 40 nm and the maximum of extinction placed at 550 nm. The GNPs obtained using 200 L of the plant extract per 2 mL of 1 mM HAuCl4 solution were bigger (up to 85 nm) and more irregular than the first ones. Their LSPR band was shifted by 10 nm towards longer wavelengths. Higher concentrations of the extract components resulted in the formation of popcorn-shaped gold nanoparticles with highly folded surface with the maximum of the extinction at 625-630 nm. The more extract was added, the bigger and less uniform gold nanoparticles were produced. The size of the GNPs from the 3rd reaction mixture varied from 45 nm to 85 nm. The increase of the extract volume in the reaction mixture resulted in a broadening of the SPR band, indicating the formation of a broad distribution of nanoparticles morphologies, which was confirmed by TEM. The popcorn-shaped gold nanoparticles obtained
ACCEPTED MANUSCRIPT using 500 L of the cistus extract per 2 mL of 1 mM HAuCl4 solution (3rd reaction mixture) were chosen for the further analysis. Synthesis of gold nanostars involved the seed preparation and further growth of the branched structures. The seed solution contained spherical, citrate-stabilized gold nanoparticles with the average size of 18 nm and the LSPR band placed at 520 nm (Fig. 2a). The nanostars formation was achieved under mild reducing conditions provided by L-ascorbic acid in the presence of silver nitrate used as a structure-directing agent. Final star-shape nanoparticles were stabilized with 2-mercaptoethanol. The obtained nanostars exhibited a broad extinction band in the range of wavelengths from 600 nm to 1200 nm, with the maximum around 870 nm (Fig. 2b). The average size of the formed nanostars was 60 nm. The synthesized star-shaped gold nanoparticles were used as a control for the further multiphoton-excited luminescence measurements.
Fig. 1. Characterization of the gold nanoparticles synthesized using cistus extract. a) Changes of the reaction mixture colours after the synthesis of the GNPs. b) Extinction spectra and c) TEM images of the obtained gold nanoparticles. Depicted volumes correspond to the extract volume added to 2 mL of 1 mM HAuCl4 aqueous solution.
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Fig. 2. Extinction spectra and TEM images of a) seed solution containing citrate-stabilized gold nanospheres and b) star-shaped gold nanoparticles stabilized with 2-mercaptoethanol.
3.2. Multiphoton-excited luminescence properties. Atomic force microscopic measurements were performed to examine the arrangement of the gold nanoparticles on the glass surface (Fig. 3). The size of the GNPs established on the basis of the AFM scans varies significantly from that determined by TEM observations, especially in the case of nanoparticles synthesized using cistus extract. TEM images revealed metal core of the nanoparticles, which exhibit high electron density, while AFM scans exposed the entire structure with the cover layer present at the surface of the GNPs. The star-shaped gold nanoparticles are stabilized with a single layer of 2-mercaptoethanol, which does not affect the size as much as the biological compounds, which may form complex structures at the surface of the biologically synthesized gold nanoparticles.
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Fig. 3. AFM scans of star-shaped and popcorn-shaped gold nanoparticles. Samples prepared using a series of the diluted GNPs solutions (10, 100 and 1000 times diluted).
It has been shown that the star-shaped gold nanoparticles exhibit multiphoton-excited luminescence properties [14]. In our experiment, we compared such properties of the chemically synthesized gold nanostars and the biologically synthesized popcorn-shaped gold nanoparticles (Fig. 4). We have chosen 1000 times diluted samples in order to distinguish signals from the single nanoparticles. The MPL intensity raster scans were obtained using the excitation wavelength set to 800 nm and the average laser power of 0.5 mW for gold nanostars and 2 mW for popcorn-shaped gold nanoparticles. The log-log scale plots of dependence of the emission intensity on the laser power revealed two-photon excited emission occurring at lower excitation laser power and higher orders of the emission process at increased excitation laser
ACCEPTED MANUSCRIPT power. The changes of the orders of the emission process from two-photon excited emission to three- and four-photon excited emission occurred at the 0.3 mW – 1 mW range of laser powers for gold nanostars and at the 1.2 mW – 2 mW range of laser powers for popcorn-shaped gold nanoparticles. Similar results were observed for the separated spherical gold nanoparticles [52], however, in our case the final excitation intensities were much higher (the designated laser powers correspond to estimated final laser intensities in the range of 30 GW/cm2 – 200 GW/cm2 [53]), indicating great photostability of the examined nanostructures. Moreover, occurrence of the three- and four-photon excited emission indicates the utility of these structures for multiphoton microscopy (applying higher orders of the emission process), which can provide better resolution than two-photon microscopy[54]. Further increasing of the laser power (above 3 mW) resulted in the loss of the emission and the degradation of the examined nanoparticles. Although higher laser power was needed for the excitation of the popcorn-shaped gold nanoparticles than for the star-shaped gold nanoparticles, we were able to record the MPL emission signals and distinguish single nanoparticles on the scans, which suggests that, similarly to the gold nanostars, the popcorn-shaped gold nanoparticles may be useful multiphoton microscopy imaging agents.
Fig. 4. MPL intensity raster scans and log-log scale dependences of emission intensity on laser power of starshaped gold nanoparticles and popcorn-shaped gold nanoparticles.
4. Conclusions. The presented work highlights the advantages of cistus extract mediated growth of gold nanoparticles. The reported procedure is simple, inexpensive and environmentally friendly. It leads to the preferential formation of popcorn-shaped GNPs, which eliminates the necessity of
ACCEPTED MANUSCRIPT further purification and separation steps. The obtained nanoparticles exhibit multiphotonexcited luminescence properties. The excitation wavelength used for the MPL measurements is located within the optical therapeutic window ensuring the maximum of the efficiency of light penetration in the tissue, which is particularly important in term of the biological samples imaging. Acknowledgements The authors acknowledge financial support from the National Science Centre Opus grant no. UMO-2013/09/B/ST5/03417. The work was also financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Science and Technology.
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Magdalena Klekotko, Katarzyna Brach, Joanna Olesiak-Banska, Marek Samoc, Katarzyna Matczyszyn PII:
S0254-0584(19)30161-0
DOI:
10.1016/j.matchemphys.2019.02.066
Reference:
MAC 21409
To appear in:
Materials Chemistry and Physics
Received Date:
25 June 2018
Accepted Date:
17 February 2019
Please cite this article as: Magdalena Klekotko, Katarzyna Brach, Joanna Olesiak-Banska, Marek Samoc, Katarzyna Matczyszyn, Popcorn-shaped gold nanoparticles: plant extract-mediated synthesis, characterization and multiphoton-excited luminescence properties., Materials Chemistry and Physics (2019), doi: 10.1016/j.matchemphys.2019.02.066
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ACCEPTED MANUSCRIPT
Popcorn-shaped gold nanoparticles: plant extract-mediated synthesis, characterization and multiphoton-excited luminescence properties. Magdalena Klekotko, Katarzyna Brach, Joanna Olesiak-Banska*, Marek Samoc, Katarzyna Matczyszyn Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wrocław, Poland E-mail: [email protected]
Highlights
Gold nanoparticles were synthesized using the aqueous extract of Cistus incanus. The biosynthesis led to popcorn-shaped gold nanoparticles with highly folded surface. The nanoparticles exhibited multiphoton-excited luminescence. Popcorn-shaped gold nanoparticles may be useful multiphoton imaging agents.
Keywords Gold nanoparticles, Biosynthesis, Multiphoton-excited luminescence properties
Abstract We describe the application of Cistus incanus extract as a source of reducing, structuredirecting and stabilizing agents in the fabrication of gold nanoparticles (GNPs) from tetrachloroauric acid solution. The synthesis resulted in the formation of nanostructures with highly extended surface area, resembling the shape of popcorn. Various ratios of plant extract volume to chloroauric acid volume were investigated in order to obtain nanoparticles with desired shapes having the narrowest size distribution. The synthesized nanoparticles were characterized using UV-Vis absorption spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy (AFM). Multiphoton-excited luminescence (MPL) properties of plant extract synthesized popcorn-shaped gold nanoparticles were examined in comparison with gold nanostars synthesized chemically. The obtained results suggest that the GNPs synthesized using cistus extract may be useful imaging agents for multiphoton microscopy.
ACCEPTED MANUSCRIPT 1. Introduction. Gold nanoparticles (GNPs) offer a wide range of useful properties (such as optical effects related to the tunable surface plasmon resonance (SPR),[1-3] catalytic activity[4-6] or light to heat conversion[7-9]), which are strongly dependent on many factors, including size[10-12] and shape[13-15] of the nanoparticles, coating of the surface[16-18] or their surroundings.[1921] The utility of these structures has been intensively explored revealing numerous applications (e.g. microscopic imaging,[22] targeted therapies,[23] biosensing[24]). Anisotropic, branched nanostructures, such as nanoflowers,[25] nanostars,[1] urchin-like nanoparticles[26] or nanodendrites[27] seem to be particularly interesting due to their highly extended surface area, numerous adsorption sites and significant enhancement of the local electromagnetic field at the branches tips, which may bring many advantages in catalysis,[28] drug delivery systems,[29] imaging[2] or surface-enhanced Raman spectroscopy (SERS).[30] This variety of the GNPs applications has contributed to the great progress in the development of numerous methods of their fabrication.[31-33] Recently, there has been growing interest in “green chemistry” approaches of the GNPs synthesis, employing bacteria[34-36], fungi[37-39] or plants[40-43]. There are multiple examples of successful application of biogenic gold nanoparticles in nanomedicine: as anticancer agents [40, 44], inflammation inhibitors[45] or antimicrobial agents[42, 43, 46]. The advantage of “green chemistry” methods in gold nanoparticle synthesis is the production of the nanostructures in eco-friendly manner without using any toxic chemicals. The stabilizing ligands of nanoparticles are usually formed from alkanoids, flavonoids, alkaloids, proteins or starch[41-43, 47]. However, most of those methods lead to the formation of nanospheres or polydisperse mixtures containing spherical, triangular and hexagonal shapes of the nanoparticles.[48] In this paper we demonstrate for the first time the use of cistus (Cistus incanus) extract for the synthesis of popcorn-shaped gold nanoparticles, characterization of the obtained structures and their multiphoton properties in comparison with star-shaped gold nanoparticles synthesized chemically. It needs to be mentioned that we have carried out many tests of the use of various plant extracts for the fabrication of gold nanoparticles and we found this particular extract to provide results that were the most interesting, because of the particular peculiar shapes of the nanoparticles produced. Cistus incanus is a herbal plant with antioxidant, anti-inflammatory and antimicrobial properties. It is mostly used in the form of infusions or dietary supplements containing its components.[49] In our work aqueous extract of Cistus incanus was used for the synthesis of popcorn-shaped GNPs. It is a simple way to produce nanoparticles without any complex protocols, toxic reagents, harmful wastes or expensive devices. Phytochemicals extracted from the plant act as reducing, structure-directing and stabilizing agents in the process of conversion of gold ions from tetrachloroauric acid solution into gold nanoparticles, however, the exact mechanism of the synthesis is still unknown. There are some assumptions, that biomolecules such as proteins, phenols and flavonoids play a key role in such a system.[48] The obtained nanoparticles exhibit multiphoton luminescence (MPL) properties, which make them potentially useful as imaging probes for multiphoton microscopy, taking advantage of the range of excitation wavelengths placed within the optical therapeutic window, where the light reaches maximal penetration depth into tissues.[50]
ACCEPTED MANUSCRIPT 2. Materials and methods. 2.1. Reagents. Tetrachloroauric(III) acid (HAuCl4•3H2O), sodium citrate tribasic dihydrate (Na3C6H5O7•2H2O), L-ascorbic acid, silver nitrate, 2-mercaptoethanol were obtained from Sigma-Aldrich, hydrochloric acid was obtained from POCH S.A. All reagents were used as received. Dried, powdered leaves of Cistus incanus were purchased in a local market. 2.2. Synthesis of gold nanoparticles. Two types of gold nanoparticles were studied: the popcorn-like ones obtained by using the cistus plant extract and the gold nanostars obtained using a more conventional protocol using typical chemicals. In both cases the gold precursor was tetrachloroauric acid, which was treated by reducing agents in order to convert it to metallic gold. It is well known that the products of such reduction can be quite diverse, depending on many details of the protocol being used. Popcorn-shaped gold nanoparticles were synthesized using water extract of cistus (Cistus incanus). The extract was prepared using 3 g of dried, powdered leaves mixed with 30 mL of distilled, deionized water. The extraction was carried out in a water bath at 100°C for 30 min. Then, the mixture was centrifuged at 5000 rpm (MPW-380 Centrifuge) for 10 min, the supernatant was filtered and stored at 4°C for further experiments. The synthesis of gold nanoparticles was performed by addition of various volumes of the extract (100 L, 200 L, 500 L, 700 L and 1 mL) to 2 mL of 1 mM HAuCl4 aqueous solution. Each of the reaction mixtures was adjusted to the final volume of 3 mL by addition of required amount of deionised water. The obtained solutions were vigorously shaken for 30 s and then left undisturbed for 24 h in the dark, at room temperature. Finally, the synthesized GNPs were centrifuged at 5000 rpm (Eppendorf Centrifuge 5418) for 5 min, supernatant was discarded and the pellet was resuspended in distilled, deionized water. Synthesis of star-shaped gold nanoparticles was based on the two-step protocol of Yuan et al.[14]. In the first step, citrate-stabilized spherical gold nanoparticles (so-called seed solution) were prepared applying the Turkevich method [51]: under vigorous stirring, 5 mL of 1% sodium citrate solution was added to 50 mL of boiling 0.5 mM HAuCl4 aqueous solution. The solution became colourless and after 15 min, ruby-red colour appeared. The solution was cooled down and used as a seed solution in the next step. The growth solution was prepared by mixing 20 mL of distilled, deionized water with 200 L of 25 mM HAuCl4 aqueous solution, 1 mL of the seed solution and 20 L of 1 M HCl solution. The star shape nanoparticles were obtained by simultaneous addition of 100 L of 100 mM ascorbic acid solution and 20 L of 25 mM silver nitrate solution. Rapid change of the mixture colour from light red to dark blue indicated the formation of branched structures. The reaction was stopped after 30 s by addition of 5 L of 2mercaptoethanol. Finally, the obtained gold nanostars were purified by centrifugation at 5000 rpm (MPW-380 Centrifuge) for 5 min and resuspension in distilled, deionized water. 2.3. Characterization of obtained nanoparticles.
ACCEPTED MANUSCRIPT The extinction spectra of the synthesized gold nanoparticles were obtained using a JASCO V670 Spectrophotometer. The measurements were performed in 10 mm quartz cuvettes in the wavelength range between 400 nm and 1200 nm. The morphological features of the GNPs were revealed using transmission electron microscopy (TEM) applying a FEI Tecnai G2 20 X-TWIN microscope. TEM sample preparation involved putting a drop of the solution of the GNPs onto carbon coated copper grids and drying at room temperature. 2.4. Multiphoton-excited luminescence properties. Samples for the multiphoton-excited luminescence measurements were prepared by spreading of 50 L of the nanoparticles solution onto cover glass. Firstly, proper concentrations of the nanoparticles were established by preparation of the samples using a series of the diluted GNPs solutions (10, 100 and 1000 times diluted) and measuring the distribution of the nanoparticles on the surface of the glass applying an atomic force microscope (AFM) (Dimensional V scanning probe microscope, Veeco) operating in the tapping mode. Multiphoton-excited luminescence properties of the gold nanoparticles were examined using a custom-made setup. The samples were placed on the XYZ piezoelectric scanning stage (TRITOR 102) and excited using a mode-locked Ti:sapphire laser (Chameleon, Coherent Inc.), with the excitation wavelength set to 800 nm. A high numerical aperture Nikon Plan Apo oil immersion objective (100×/1.4 NA), working in epifluorescence mode, was used for the focusing of an incident laser beam on the sample and for the collecting of the multiphoton-excited emission. The emitted signal was separated from the incident light on a dichroic mirror (cutoff wavelength 720 nm) and collected by avalanche photodiodes operating in the photon counting regime. 3. Results and discussion. 3.1. Synthesis and characterization of gold nanoparticles. Addition of the cistus extract to 1 mM tetrachloroauric acid aqueous solution leads to a rapid change of the reaction mixture colour from light yellow to a range of colours from purple to dark blue, depending on the extract volume (Fig. 1a). The extinction spectra recorded after the synthesis revealed characteristic surface plasmon resonance bands of the obtained gold nanoparticles (Fig. 1b). The morphology of the synthesized GNPs was determined under transmission electron microscope (Fig. 1c). The first reaction mixture (100 L of the cistus extract per 2 mL of 1 mM HAuCl4 solution) contained mostly spherical nanoparticles with the size up to 40 nm and the maximum of extinction placed at 550 nm. The GNPs obtained using 200 L of the plant extract per 2 mL of 1 mM HAuCl4 solution were bigger (up to 85 nm) and more irregular than the first ones. Their LSPR band was shifted by 10 nm towards longer wavelengths. Higher concentrations of the extract components resulted in the formation of popcorn-shaped gold nanoparticles with highly folded surface with the maximum of the extinction at 625-630 nm. The more extract was added, the bigger and less uniform gold nanoparticles were produced. The size of the GNPs from the 3rd reaction mixture varied from 45 nm to 85 nm. The increase of the extract volume in the reaction mixture resulted in a broadening of the SPR band, indicating the formation of a broad distribution of nanoparticles morphologies, which was confirmed by TEM. The popcorn-shaped gold nanoparticles obtained
ACCEPTED MANUSCRIPT using 500 L of the cistus extract per 2 mL of 1 mM HAuCl4 solution (3rd reaction mixture) were chosen for the further analysis. Synthesis of gold nanostars involved the seed preparation and further growth of the branched structures. The seed solution contained spherical, citrate-stabilized gold nanoparticles with the average size of 18 nm and the LSPR band placed at 520 nm (Fig. 2a). The nanostars formation was achieved under mild reducing conditions provided by L-ascorbic acid in the presence of silver nitrate used as a structure-directing agent. Final star-shape nanoparticles were stabilized with 2-mercaptoethanol. The obtained nanostars exhibited a broad extinction band in the range of wavelengths from 600 nm to 1200 nm, with the maximum around 870 nm (Fig. 2b). The average size of the formed nanostars was 60 nm. The synthesized star-shaped gold nanoparticles were used as a control for the further multiphoton-excited luminescence measurements.
Fig. 1. Characterization of the gold nanoparticles synthesized using cistus extract. a) Changes of the reaction mixture colours after the synthesis of the GNPs. b) Extinction spectra and c) TEM images of the obtained gold nanoparticles. Depicted volumes correspond to the extract volume added to 2 mL of 1 mM HAuCl4 aqueous solution.
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Fig. 2. Extinction spectra and TEM images of a) seed solution containing citrate-stabilized gold nanospheres and b) star-shaped gold nanoparticles stabilized with 2-mercaptoethanol.
3.2. Multiphoton-excited luminescence properties. Atomic force microscopic measurements were performed to examine the arrangement of the gold nanoparticles on the glass surface (Fig. 3). The size of the GNPs established on the basis of the AFM scans varies significantly from that determined by TEM observations, especially in the case of nanoparticles synthesized using cistus extract. TEM images revealed metal core of the nanoparticles, which exhibit high electron density, while AFM scans exposed the entire structure with the cover layer present at the surface of the GNPs. The star-shaped gold nanoparticles are stabilized with a single layer of 2-mercaptoethanol, which does not affect the size as much as the biological compounds, which may form complex structures at the surface of the biologically synthesized gold nanoparticles.
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Fig. 3. AFM scans of star-shaped and popcorn-shaped gold nanoparticles. Samples prepared using a series of the diluted GNPs solutions (10, 100 and 1000 times diluted).
It has been shown that the star-shaped gold nanoparticles exhibit multiphoton-excited luminescence properties [14]. In our experiment, we compared such properties of the chemically synthesized gold nanostars and the biologically synthesized popcorn-shaped gold nanoparticles (Fig. 4). We have chosen 1000 times diluted samples in order to distinguish signals from the single nanoparticles. The MPL intensity raster scans were obtained using the excitation wavelength set to 800 nm and the average laser power of 0.5 mW for gold nanostars and 2 mW for popcorn-shaped gold nanoparticles. The log-log scale plots of dependence of the emission intensity on the laser power revealed two-photon excited emission occurring at lower excitation laser power and higher orders of the emission process at increased excitation laser
ACCEPTED MANUSCRIPT power. The changes of the orders of the emission process from two-photon excited emission to three- and four-photon excited emission occurred at the 0.3 mW – 1 mW range of laser powers for gold nanostars and at the 1.2 mW – 2 mW range of laser powers for popcorn-shaped gold nanoparticles. Similar results were observed for the separated spherical gold nanoparticles [52], however, in our case the final excitation intensities were much higher (the designated laser powers correspond to estimated final laser intensities in the range of 30 GW/cm2 – 200 GW/cm2 [53]), indicating great photostability of the examined nanostructures. Moreover, occurrence of the three- and four-photon excited emission indicates the utility of these structures for multiphoton microscopy (applying higher orders of the emission process), which can provide better resolution than two-photon microscopy[54]. Further increasing of the laser power (above 3 mW) resulted in the loss of the emission and the degradation of the examined nanoparticles. Although higher laser power was needed for the excitation of the popcorn-shaped gold nanoparticles than for the star-shaped gold nanoparticles, we were able to record the MPL emission signals and distinguish single nanoparticles on the scans, which suggests that, similarly to the gold nanostars, the popcorn-shaped gold nanoparticles may be useful multiphoton microscopy imaging agents.
Fig. 4. MPL intensity raster scans and log-log scale dependences of emission intensity on laser power of starshaped gold nanoparticles and popcorn-shaped gold nanoparticles.
4. Conclusions. The presented work highlights the advantages of cistus extract mediated growth of gold nanoparticles. The reported procedure is simple, inexpensive and environmentally friendly. It leads to the preferential formation of popcorn-shaped GNPs, which eliminates the necessity of
ACCEPTED MANUSCRIPT further purification and separation steps. The obtained nanoparticles exhibit multiphotonexcited luminescence properties. The excitation wavelength used for the MPL measurements is located within the optical therapeutic window ensuring the maximum of the efficiency of light penetration in the tissue, which is particularly important in term of the biological samples imaging. Acknowledgements The authors acknowledge financial support from the National Science Centre Opus grant no. UMO-2013/09/B/ST5/03417. The work was also financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Science and Technology.
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