Biotemplating Plasmonic Nanoparticles Using Intact Microfluidic

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Biotemplating Plasmonic Nanoparticles using Intact Microfluidic Vasculature of Leaves Karthik Pushpavanam, Sanjitarani Santra, and Kaushal Rege Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5041568 • Publication Date (Web): 02 Nov 2014 Downloaded from http://pubs.acs.org on November 8, 2014

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Biotemplating Plasmonic Nanoparticles using Intact Microfluidic Vasculature of Leaves Karthik Pushpavanam1, Sanjitarani Santra2, Kaushal Rege1*

1

Chemical Engineering, Arizona State University, Tempe, AZ 85287-6106, USA

2

Material Science & Engineering, Arizona State University, Tempe, AZ 85287-6106, USA

KEYWORDS: Biotemplating, nature-inspired, leaf venation, catalysis, gold nanoparticles, silver nanoparticles

ACS Paragon Plus Environment

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ABSTRACT Leaves are an abundant natural resource, and consist of a sophisticated microfluidic network of veins that transports nutrients and water, thereby enabling photosynthesis. Here, we simultaneously exploit the microfluidics as well as chemistry of processed leaf vasculature (venation) in order to template the in situ generation of plasmonic metal (gold and silver) nanoparticles under ambient conditions. This biotemplating approach involves capillary flow of metal salts through skeleton leaf vasculature, and does not require additional reducing agents for plasmonic nanoparticle formation. Gold nanoparticles, 30-40 nm in diameter, and silver nanoparticles, approximately 9 nm in diameter, were formed within the intact leaf vasculature using this method. Absorption spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction analyses were employed to ascertain the formation of nanoparticles in the leaf veins. Fourier transform infrared (FT-IR) spectroscopy was employed in order to obtain insights into functional groups responsible for formation of the plasmonic nanoparticles within the leaves. Gold nanoparticles, templated within leaves, demonstrated excellent catalytic properties, thereby imparting catalytic and plasmonic properties to the leaf itself. Furthermore, nanoparticles can be recovered from the leaves as soluble dispersions by simply combusting the organic leaf matter. Taken together, this is a simple yet powerful biotemplating approach for the generation of plasmonic nanoparticles and formation of biotic-abiotic structures for diverse low-cost applications in sensing, catalysis and medicine.


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INTRODUCTION

Size-dependent properties1, 2, 3 of plasmonic nanoparticles have made them attractive materials for several applications in sensing, catalysis, energy technologies, and medicine.4, 5, 6, 7, 8, 9 While gold nanoparticles have been investigated as contrast agents in imaging, photothermal convertors for energy and medical applications, and plasmonic biosensors, silver nanoparticles have been extensively investigated in antibacterial, biosensing, and therapeutic applications. However, synthesis of plasmonic nanoparticles often relies on toxic and expensive chemicals, or on harsh experimental conditions.10, 11, 12 Nature offers a plethora of templates for the ‘green synthesis’ of novel materials, including nanoparticles.

13, 14, 15

In particular, the wide availability of plant

biomass and extracts makes them a valuable resource for nanoparticle synthesis. 16, 17, 27

Leaves, an abundant natural resource, contain a sophisticated microfluidic vasculature network, which is employed in the transport of nutrients and water. Figure S1 shows scanning electron microscopy (SEM) image of intact vasculature of a processed skeleton leaf, showing extensive interconnects and pitted veins. Leaf vasculature / venation is made largely of lignin,18 a heterogeneous and amorphous complex polymer. Lignin contains oxidizable functional groups (e.g. –OH and -CHO) that possess both, reducing and stabilizing properties. We hypothesized that these properties can be exploited for facilitating nanoparticle formation.

Here, we exploit the microfluidics and chemistry of skeleton leaf venation in order to template the in situ formation of plasmonic nanoparticles from metal salts, without the need for additional reducing agents. Transport of the salt solution through the skeletal leaf venation is facilitated by


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capillary flow, and reduction of the metal ions results in formation and immobilization of nanoparticles within the veins of the three dimensional vasculature. Indeed, eucalyptus trees have been shown to naturally transport dissolved gold all the way from the roots to the leaves19. Nanoparticles formed within the skeleton leaf vasculature are accessible to external reagents, do not leach out of the vasculature, and demonstrate excellent catalytic properties. This leaf vasculature-based biotemplating approach is a simple, green synthesis method that can lead to novel lower-cost platforms and devices for applications in sensing, diagnostics, and catalysis.

EXPERIMENTAL Materials: Gold(III) chloride trihydrate (HAuCl4.3H20), sodium borohydride (NaBH4) and pnitrophenol were purchased from Sigma-Aldrich. Silver Nitrate (AgNO3) was purchased from Fisher Scientific, and quantum dots were purchased from Invitrogen. Nanopure water (resistivity, 18.2 MΏ-cm) was used in all syntheses. Bare processed leaves with a bleached white appearance (Product Name: Skeleton Leaves Large 10 inch Magnolia Leaves; 100 leaves) were purchased from Save on Crafts Inc. All components were used without further processing / purification.

Confocal Microscopy and Flow Analysis: CdSe quantum dots (2 ml) were subjected to flow through the venation by means of capillary action, in order to visualize the microfluidic flow within the leaf. After one hour, sections (10 mm * 10 mm) of the leaf were cut and incubated at room temperature with 2% paraformaldehyde (fixative) for 30 minutes on cover slips with gentle rocking. The fixative was removed and the sections were rinsed with nanopure water. The section, along with the cover slips, was mounted on top of glass slides with mounting media (9 parts glycerol and 1 part 10X PBS by volume), and the cover slip was sealed using nail polish. The sections were excited at 520 nm and emission was recorded at 620 nm with a Leica SP2


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confocal microscope. The fluorescence intensity was quantified at different depths using ImageJ software. The background noise was removed prior to analysis and the mean value was reported. The fluorescence intensity was normalized with the maximum intensity of the specific sample in order to compare different regions of the leaf.

Formation of Plasmonic Nanoparticles inside Leaf Veins: Gold and silver salt solutions (1x10-2 M concentration) were prepared by dissolving HAuCl4.3H20 and AgNO3, respectively, in nanopure water. The salt solutions (2mL) were placed in caps of 50 mL centrifuge tubes, and the petiole of a bare leaf was dipped inside the solution (Figure 1A). Only the petiole was in contact with the solution in order to facilitate capillary flow through the leaf venation. Gold nanoparticle formation was seen as early as 10 minutes upon dipping, while the silver nanoparticle formation took a longer time (> 24 hours).

Recovery of Silver Nanoparticles from Leaf Veins: The tip of the leaf was combusted completely using a bench-top Bunsen burner, and the burnt contents were added to an aqueous solution of 1.5 ml nanopure water in order to enable dispersion of the silver nanoparticles. Following sedimentation of the ash and filtration through a 0.45 µm filter, the supernatant (nanoparticle dispersion) was concentrated to approximately 200 µL and used for further characterization.

Characterization of Nanoparticle-Containing Leaves and Recovered Nanoparticles: Absorbance of the composite was measured using a BioTek Synergy 2 plate reader in concert with Gen5 software. The absorbance spectrum was read with a step size of 5nm from 300nm to 999nm. Nanopure water was used as the blank in all cases. Fluorescence of silver nanoparticles


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recovered following combustion of the leaves was measured at an excitation wavelength of 360 nm and an emission of 485 nm. The blank used in this case was a burnt bare skeleton leaf and silver nitrate solution of same molarity used in the experiment. Scanning electron microscopy, (SEM) FEI XL 30, was used in the secondary electron mode in order to study the surface morphology of the plasmonic leaf. The leaf was carbon coated and then used for imaging using SEM. Nanoparticle-containing ‘plasmonic’ leaves were milled and loaded onto copper grids in order to facilitate visualization of the nanoparticles using transmission electron microscopy (TEM; JEOL 2010 F); TEM visualization of silver nanoparticles in dispersion was carried out using a CM200–FEG instrument. Sizes (diameters) of gold and silver nanoparticles were determined manually from microscopy images using ImageJ software. All SEM and TEM studies were carried out at the Leroy Eyring Center for Solid State Sciences at ASU.

FTIR Spectroscopic Analysis of Bare and Nanoparticle-containing leaves: Each leaf (bare and nanoparticle-containing) was milled for 30 minutes in a Ball mill – Spex – Certiprep 8000 D. The resultant powder was mixed with KBr and pressed into a pellet which was further analyzed by FT-IR/FT-Raman (Bruker IFS 66V/S). Scanning was carried out from 4000 cm-1 to 400 cm-1 with 128 scans for each sample.

Catalytic Activity of Gold Nanoparticle-containing Leaves: Leaves with gold nanoparticles embedded in their veins were investigated for the catalytic reduction of p-nitrophenol to paminophenol. Briefly water (1.4 mL), 2mM p-nitrophenol (0.3 mL) and 30 mM NaBH4 (1 mL) were used as the reaction mixture, and the reaction was carried out in a 2 mL cuvette. Two sections, of size 10 mm * 10 mm each, were cut from a nanoparticle-containing leaf, and added


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to the reaction solution mixture. The reaction was followed spectroscopically by measuring a decrease in the 400 nm peak, which was indicative of disappearance of p-nitrophenol and formation of p-aminophenol. The reaction solution mixture in presence of the bare leaf (i.e. without gold nanoparticles present in the venation) was used as the control. The reaction was deemed to proceed to completion when the reaction mixture turned colorless visually. Sodium borohydride was used in excess to allow rate constant calculation by assuming pseudo first order kinetics. The slope in the plot of ln(A/A0) vs. time (min) was determined using Microsoft Excel, and reported as a pseudo first order rate constant for the catalysis reaction.

The reusability of gold nanoparticle-containing leaves as catalysts was determined using a dip procedure. After every reaction cycle (described above), the leaf section was removed and washed with water to remove any reactant / products that may have been physically adsorbed. The leaf sections were reused for four additional cycles (total five cycles), and rate constants were calculated as described above for each cycle. The rate constant of each recycle was compared to that of the first, and statistical differences between these were determined using a paired t-test comparison; a p-value < 0.05 is considered to be statistically significant.

RESULTS AND DISCUSSION The abundance of leaves as a natural resource makes them attractive candidates for biotemplating nanoparticles using potentially greener and lower cost methods. We hypothesized that capillary flow through the leaf vasculature could facilitate nanoparticle formation inside leaf veins. Microfluidic devices have the potential to facilitate generation of nanoparticles with higher fidelities compared to other batch processes20 21. We therefore hypothesized that the use


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of leaf venation microfluidics can have some distinct advantages compared to other batch processes that use leaf extracts22, including stand-alone, low-cost plasmonic platforms that may be used as devices in subsequent studies. The following sections describe the use of venation microfluidics for generation of gold and silver nanoparticles, as a potential first step eventually leading towards leaf-based ‘devices’ for diverse applications including catalysis.

Capillary Flow within Leaf Vasculature. We first investigated if fluid transport due to capillary action was possible inside bare skeleton leaves. Scanning electron microscopy (SEM) imaging revealed presence of several micropores in the leaf vasculature (Figure S1 A & B); specifically, pitted xylem structures,23 which facilitate transport, can be visualized (Figure S1 C). Visual observation of liquid flow through the microfluidic leaf vasculature indicated that the flow is faster at initial time points and decelerates over time. It can be assumed that the leaf is initially saturated with a non-wetting fluid (i.e. air). When a wetting fluid (e.g. salt solution) is introduced at the petiole of the leaf (Figure 1; please see experimental section for details), the capillary pressure draws the liquid into the pores of the leaves. Since the transport path of the wetting fluid is upward into the leaf venation, the flow likely ceases when gravitational forces acting on the column of fluid balance the capillary force that transports the liquid upwards through the pores. Initially the liquid rises much faster since the trapped air inside the venation can escape to the atmosphere.24 However, as the liquid rises further, escape of the compressed air is likely difficult, which results in resistance to further flow of the liquid upward into the leaf venation.24 As a result, partial penetration of the liquid within the skeleton leaf vasculature, and subsequent nanoparticle formation (please see section titled Biotemplating Nanoparticle Formation below) can be seen in these regions (Figure 1).


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In order to further investigate liquid flow through the leaf venation, aqueous dispersions of commercially available (pre-made), red-fluorescent (emission = 655 nm) quantum dots (QDs) were subjected to capillary transport. After one hour, different sections of the leaf were cut and visualized using confocal fluorescence microscopy (Figure 2A; see experimental section for details). The two sections shown in Figure 2A.1 and 2A.3 are representative images of the extreme ends of the Z stack of the leaf venation. The fluorescence intensity of quantum dots was significantly less at the top (Z stack = 50) and bottom (Z stack = 0) of the fluidic leaf channel, compared to that seen in the middle sections (Figure 2A.2); a schematic depicting this radial distribution of QDs is shown in Figure 2B. The fluorescence intensities were normalized to the maximum fluorescence intensity obtained, and plotted to show the radial distribution of QDs in the leaf channel (Figure 2C). The resultant flow is quasi-parabolic in nature with the maximum fluorescence intensity in the middle section of the leaf. This flow profile is similar to HagenPoiseuille flow conditions, and has been previously seen in other similar systems.25 In addition to facilitating visualization of liquid transport, these studies indicate that the microfluidic leaf network can also transport dispersions containing pre-made nanoparticles.

Biotemplating Plasmonic Nanoparticles inside Leaf Veins. Following a visualization of transport in the leaf vasculature, we next employed the leaf venation for biotemplating plasmonic metal (gold / silver) nanoparticles. In this approach, a metal salt solution (2 mL, 1x10-2 M HAuCl4.3H20 for gold or 2 mL, 1x10-2 M AgNO3 for silver), was introduced through the microfluidic network of leaf venation network by means of capillary action at room temperature (Figure 1). Metal salts were reduced in situ, and the resulting nanoparticles were held in place,


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likely by lignin, inside the leaf vein. Maroon coloration could be observed as early as 10 minutes following the start of gold salt transport through the bare leaf venation, resulting in the formation of ‘plasmonic leaves’. The maroon coloration is a visible indicator of gold nanoparticle formation, and is different from the colorless or pale yellow nature of the gold salt solutions.26 The color intensity was further enriched at 2h, indicating increased flow as well as activity of gold nanoparticle formation over time (Figure 1 C).

Plasmonic properties of the leaf with gold nanoparticles embedded inside the vasculature (Figure 3 A; see experimental section for details), were verified by first determining the optical absorbance of mechanically crushed leaf sections in water; a distinct absorption peak at 520 nm, indicative of gold nanosphere formation was seen (Figure 3 B). Gold nanospheres (mean size = 35.7 nm; standard deviation = 16.1 nm; n = 47 individual nanoparticles) in the leaf venation were visualized using Scanning Electron Microscopy (SEM); nanoparticles can be seen throughout the vein (Figure 3 C). The leaf containing gold nanoparticles was milled for further analysis using transmission electron microscopy (TEM) (Figure 3 D). We carried out structural analysis on nanoparticles using electron diffraction in order to confirm the presence of metallic nanoparticles. Analysis of the diffractogram, shown as inset in Figure 4D, led to the identification of (111), (002), (022) and (222) d-spacings, in agreement with the standard database of International Centre for Diﬀraction Data (ICDD). Taken together, these findings confirm the formation of gold nanoparticles within leaf veins. Nanoparticle distribution in different regions of the leaf was calculated using absorbance as a surrogate for concentration. Approximately 1cm * 1 cm regions of the leaf, highlighted with red-colored rectangular boxes in Figure S2, were cut and their absorbance values were determined using a UV-Vis


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spectrophotometer. The reported absorbance values were calculated by subtracting the peak absorbance value and the baseline (shown in inset of Figure S2). As seen from Figure S2, the absorbance values were relatively similar in the four different regions inspected. This suggests near-uniformity of nanoparticle distribution in different sections of the leaf.

We next investigated the formation of silver nanoparticles using the skeleton leaves. The kinetics of silver nanoparticle formation were much slower compared to those of gold nanoparticles; silver nanoparticles were formed over 24 h (Figure 4 A; please see experimental section for details), while gold nanoparticle formation was seen as early as 10 minutes. Furthermore, the yield of silver nanoparticles was not as high as that observed with gold. This required recovery of silver nanoparticles by decomposing the leaf matter by combustion, and subsequent concentration as described in the Experimental section. Absorbance analysis of silver nanoparticles recovered from the leaves demonstrated a broad spectrum (Figure 4 B), which is likely due to absorbance of both, lignin (peak absorption wavelength = 360 nm),27 and silver nanoparticles (peak absorption wavelength = 420 nm). These studies also indicate that degrading the leaf organic matter (e.g. by combustion) is a straightforward method for recovering plasmonic nanoparticles as dispersions, without affecting their integrity.

Following recovery from the leaf, silver nanoparticle dispersions demonstrated a fluorescence signal at 485 nm wavelength, when excited at 360 nm wavelength (fluorescence intensity of silver nanoparticle suspension = 26073.25 fluorescence units, standard deviation = 11300.45 fluorescence units, n = 4),27,

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while a dispersion containing the decomposed leaf and silver

nitrate salt demonstrated very little fluorescence (969 fluorescence units). The high signal


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intensity in the case of silver nanoparticles recovered from the leaf is likely due to the enhancement of the lignin fluorescence.29, 30 This observation of plasmon-enhanced fluorescence indicates the proximity of lignin to the silver nanoparticles, likely as a core (silver) - shell (lignin) nanoparticle.

TEM visualization was carried out in order to visualize the silver nanoparticles recovered after combustion of the leaf organic matter. Figure 4 C confirms the formation of silver nanoparticle cores of mean diameter = 8.9 nm, with a standard deviation = 1.2 nm (based on n = 48 individual particles). In order to verify that combustion of the leaf organic matter did not result in silver nanoparticle formation, silver nanoparticle-containing leaves were milled and analyzed for selective area diffraction using TEM (Figure 4 D); the leaves were not decomposed by combustion for these imaging studies. Analysis of the diffractogram pattern led to the identification of (100), (102), (112) and (213) d-spacings, which correspond to hexagonal-closepacking (hcp) crystals, in agreement with the ICDD database.

Our observation that formation of gold nanoparticles occurs at a faster rate than silver nanoparticles is similar to those seen in other reports in the literature.31. This difference is most likely due to the difference in redox potential between the two metal ions32; Ag+ has a lower redox potential when compared to Au3+, and therefore, thermodynamically the reduction of silver(I) is not as favorable as that of gold(III). Kinetically, the growth of gold and silver colloids has been proposed to proceed via different routes33, 34. Gold nanoparticle formation first involves reduction of the metal ions to zerovalent nuclei, which then undergo coalescence to form larger particles. The third step in the growth process proceeds through diffusion which is maintained by


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continuous reduction of the metal salt. This is followed by rapid aggregation / coalescence of the particles to yield the final nanoparticle size which depends on the initial concentration of the precursor salt solution.

While the growth of both, gold and silver nanoparticles has been shown to proceed through coalescence, the major difference between these is that a two-step coalescence process has been shown to exist for silver nanoparticle growth, while a one-step process has been proposed for gold nanoparticle growth. The first step involves reduction of metal ions to small sized nanoparticles which is common to both metal salts. The second step, characteristic of silver nanoparticles, is a prolonged metastable state which could arise due to the strong interaction of macromolecules in the leaf with the silver nanoclusters; such a metastable state and subsequent interactions with macromolecules have been shown in poly(vinylpyrrolidone) and polyacrylic acid systems33,

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. Hexagonal-close packing of silver nanoparticles has also been previously

observed in such systems36. The final step is the coalescence of these metastable particles to a particle size where colloidal stability is reached. The additional coalescence step and the thermodynamically unfavorable reaction involving the reduction of silver salt to nanoparticles are likely responsible for the slower kinetics of silver nanoparticle formation, and the concomitant delay in visual identification of silver nanoparticles that are formed inside the leaf venation, compared to observations in case of gold nanoparticles.

Infrared spectroscopy has been widely used to identify the presence of functional groups that may facilitate nanoparticle formation

37, 38

. The intact skeleton leaf (i.e. in absence of metal

nanoparticles) shows presence of bands at 3421 cm-1 and 1043 cm-1 which correspond to


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hydroxyl groups 39. C-O and C=O stretching modes of the carboxylic groups were seen at 1326 cm-1 and 1774 cm-1, respectively (Figure 5) 40. The amide I and amide II bands appeared at 1600 cm-1 and 1506 cm-1, which should be expected given the natural source of the template (i.e. leaf)32. Presence of the C-N stretching mode at 1370 cm-1 further indicates the presence of amines in the intact leaf. Taken together, the bare intact leaf has an abundance of hydroxyl, carbonyl, carboxyl and amine functional groups, which is along expected lines for this biological system. More importantly, these groups are known to reduce metal salts, as well as stabilize metal nanoparticles32, 41.

We next investigated FT-IR spectra of leaves in which, gold and silver nanoparticles were present in situ, following complete reduction and nanoparticle formation. We hypothesized that shifts in the peak position, coupled with their respective intensities, may help identify moieties that play a role in reducing metal ions for nanoparticle formation31. The increase in the intensity of the bands coupled with their respective shifts (5 ± 25 cm-1) are indicators of the role of these functional groups in reducing and stabilizing nanoparticles42, 43. Increase in transmittance in the C-O and C=O stretching modes of the carboxylic groups and the amide I and amide II bands were also observed. This suggests possible capping by carboxylate ion (-COO-) and the lone pair electrons on the free amine groups, which can assist the reduction of metal salts to their respective nanoparticles44,

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. An increase in the C-N stretching mode can be visually

identified which would further suggest the role of the amine functional groups in the stabilization of the nanoparticles44. Increase and shifts in the bands corresponding to hydroxyl groups can imply their consumption, and most likely assistance in the stabilization of the nanoparticles through adsorption on surfaces47,

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. Functional groups including amines, hydroxyls, and


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carboxylic acids are expected to be present in the leaf venation47, and are most likely to be responsible for the in situ formation of gold and silver nanoparticles.

Catalytic Activity of Nanoparticle-Containing Leaves. We next investigated if leaves with the plasmonic nanoparticles embedded in the venation can be employed for practical applications. Para (p)-nitrophenol is an intermediate in the synthesis of paracetamol,49 but is considered as an environmental pollutant50,

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. Reduction of yellow-colored solutions of p-nitrophenol to the

colorless p-aminophenol using sodium borohydride as the reducing agent (Figure 6 A) is a key step in remediation of the former, and also an intermediate step in the synthesis of paracetamol (obtained subsequently by acetylation of p-aminophenol)49. Although this reaction is thermodynamically favorable, slow kinetics of reduction necessitate the use of catalysts in practical applications.52 Gold nanoparticles act as intermediary between sodium borohydrate and p-nitrophenol, and facilitate electron transfer which overcomes the kinetic barrier to reduction, thereby facilitating the reaction.

We therefore investigated if the gold nanoparticle-containing skeleton leaf can be employed for the catalytic conversion of p-nitrophenol to p-aminophenol in presence of sodium borohydride at room temperature. Two sections (10mm * 10mm each) of the plasmonic skeleton leaf were added to a solution of p-nitrophenol in water along with sodium borohydrate, and the reaction was monitored using UV-visible spectroscopy. The decrease in the intensity of the 400 nm absorbance peak of p-nitrophenol was used as an indicator of reaction progress, leading to the formation of p-aminophenol, a colorless product easily visible to the naked eye (Figure 6 B; please see experimental section for details). Following disappearance of the yellow color, the


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plasmonic leaf sections were removed from the reaction container, and the liquid was assayed using UV-visible spectroscopy. The absence of a peak at 400 nm indicated conversion of pntirophenol to p-aminophenol, and the absence of a peak at ca. 520 nm indicated that gold nanoparticles had not leached out of the leaf skeleton into the reaction mixture. In addition, skeletal leaf sections without gold nanoparticles did not engender a change in the yellow color of the p-nitrophenol solution, indicating that bare leaves (without the presence gold nanoparticles) lack of catalytic activity.

In order to further investigate the kinetics of the catalytic reaction, the absorbance (A) of the reaction solution was tracked as a function of time, and normalized to the absorbance immediately before addition of the plasmonic leaf sections (A0); the reaction was assumed to have proceeded to completion when the yellow color disappeared completely. We assumed firstorder kinetics given the excess use of sodium borohydride in this reaction, and plotted ln(A/A0) as a function of time for six independent reactions (Figure 6 C); the slope of this line yields the first-order rate constant of the reaction.52, 53 It is interesting to note that the values of the kinetic rate constants obtained for the gold-catalyzed conversion of p-nitrophenol using plasmonic leaf sections are comparable to the ones seen in literature with other systems.52, 54, 55 Taken together, these results show that the pores of the skeleton leaf, as well as the gold nanoparticles, were accessible to the reaction mixture, and the accessibility of the gold nanoparticles was responsible for the catalytic activity of the plasmonic leaf.

The reusability of the gold nanoparticle-containing leaf catalyst was investigated to further demonstrate the utility of this approach. The solid phase catalyst can be easily removed from the


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liquid phase reaction mixture and re-used with a fresh reaction mixture (please see experimental section for more details). The kinetic rate constant calculated for the first cycle was used to normalize the rate constants calculated for the subsequent four recycles of p-nitrophenol reduction. As seen in Figure 7, the leaf-based catalyst could be reused three times without significant loss of activity. A significant decrease in the catalytic activity was observed only during the fifth cycle. Similar limitations with catalyst reusability for reduction of p-nitrophenol have been demonstrated in literature56.

To our knowledge, this is the first report on the direct use of only the intact leaf skeleton for the generation of plasmonic nanoparticles. Previous reports have described the use of leaf extracts for the generation of gold nanoparticles22. The use of a natural microfluidic platform can facilitate the further development of low-cost devices using embedded plasmonic nanoparticles, something that will not be possible using soluble leaf extracts. The venation-based platform can be employed in sensing / detection and catalytic applications, using microfluidics-based analyte detection / conversion. The leaf skeleton has been widely accepted to be largely composed of composition of lignin, which is in contrast to the complex composition that may be expected for leaf extracts23, 57. Such potential complex compositions may make it difficult to precisely control the fidelity and yield of nanoparticle formation58. Further, the use of liquid-phase dispersions will likely require additional steps, e.g. centrifugation, for separating the products from the nanoparticles59. Presence of nanoparticle embedded inside the solid-phase leaf venation obviates this requirement.


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CONCLUSIONS In thus study, we have demonstrated that the intact microfluidic vasculature / venation network of skeleton leaves can be employed for the facile generation of ‘plasmonic leaves’ without the need for additional reducing agents. The process is carried out at room temperature in all cases, which further indicates advantages of this ‘green chemistry’ approach. Skeletal leaves containing gold nanoparticles were employed for the catalytic conversion of p-nitrophenol to paminophenol, indicating that the gold nanoparticles and the microfluidic venation are accessible, active, and reusable. Nanoparticles can also be recovered as stable dispersions by simply decomposing the leaf organic matter by combustion of the organic matter, as was demonstrated in case of the silver nanoparticles formed inside the leaf veins. Taken together, the use of leaf vasculature microfluidics and chemistry is a novel approach for bio-templating organic-inorganic interfaces, and can be a first step for low-cost nature-inspired platforms for sensing, catalysis, and medicine.16, 60


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FIGURES

C A

B

0 min

C

30 min

2h

Figure 1. Visible changes in (A) bare leaf skeleton following capillary flow of gold salt solution through leaf venation after (B) 30 minutes and (C) 2 hours, resulting in the formation of ‘plasmonic leaves’. The visible maroon coloration is indicative of gold nanoparticle formation inside the leaf venation, and the petiole is shown by an arrow in (A).


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A.1

A.2

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A.3

A.4

flow

! B

C

Figure 2. Characterization of leaf venation microfluidics using confocal microscopy of B fluorescent CdSe quantum dots (excitation at 520 nm and emission at 620 nm wavelength) at different Z sections inside veins (n = 4). A.1, A.2 and A.3 correspond to Z stack numbers 3, 26 and 41, respectively, in the microscopy image; 3 and 41 are representative sections of the upper and lower sections of the microfluidic leaf, channel while 26 is indicative of the middle section (i.e. radial center). Confocal microscopy image of the middle section of a bare leaf in absence of quantum dots is shown in A.4; no fluorescence is seen as expected. (B). Schematic depicting the distribution of quantum dots in the leaf venation (C). A quasi-parabolic profile of the normalized fluorescence intensity, plotted against the Z stack numbering, indicates characteristics of laminar flow through the venation.


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A

A

B

B

D

C

D

50 nm

Figure 3. (A). Skeleton leaf with maroon-colored region, indicative of presence of gold nanoparticles (B). UV-visible absorption spectrum of gold nanoparticles. The spectrum was obtained after mechanically crushing the leaf and resuspending the contents in water. The absorbance spectrum shows a peak at 520 nm (arrow), characteristic of gold nanospheres. (C). SEM image of gold nanoparticles inside the leaf venation. Scale bar = 1 µm. (D). TEM image of gold nanoparticles with the diffractogram pattern indicating presence of (111), (002), (022), and (222) d-spacings, which correspond to face centered cubic crystal structure of the analyzed gold nanoparticle (shown inside the red box in D). Scale bar = 50 nm.


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A

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B

A

B

302

DD

C

10 nm

Figure 4. (A). Visual observation of silver nanoparticles in the skeleton leaf. The red arrow in the image indicates a pale yellow-colored region where silver nanoparticles are formed in the leaf. (B). UV-visible absorption spectrum of silver nanoparticle dispersions (please see Experimental section for details). The spectrum shows peak at 420 nm, which is characteristic for silver nanoparticles, and also a peak at 360 nm which corresponds to lignin absorbance. (C). TEM image of silver nanoparticles recovered after decomposing the leaf organic matter by combustion. Scale bar = 20 nm. (D). TEM image before recovery of silver nanoparticles by combustion of leaf organic matter (i.e. silver nanoparticles present inside leaf venation). Analysis of the diffraction pattern of the imaged area indicates presence of (100), (102), (112) and (213) d-spacings, which correspond to hexagonal-close packing of the analyzed area of silver nanoparticles. Scale bar = 10 nm.


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Figure 5. FT-IR spectroscopic analysis of bare and plasmonic leaf samples, with bare leaf spectra denoted by solid black line and the spectra of gold and silver nanoparticle-containing leaves denoted by a dashed and dotted line respectively.


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A. Au

B.

C.

Figure 6. Catalytic conversion of p-nitrophenol to p-aminophenol using gold nanoparticlecontaining plasmonic leaf sections. (A) Schematic of the conversion of p-nitrophenol to paminophenol. (B) The reaction was carried out in a plastic vial with the control (left) being the reaction mixture with a “bare” leaf, and the experiment (right) containing the mixture with the plasmonic leaf section (please see Experimental section for details). Disappearance of the yellow color of p-nitrophenol is used as a visual indicator for the completion of the catalytic reaction. (C). Plot of ln(A/A0) vs. reaction time (min); A is the absorbance at a particular time point, while A0 is the absorbance at time = 0 min. The slope of this plot gives the value of the corresponding rate constant in each case. Each curve denotes one independent experiment; a total of n = 6 independent experiments were carried out on different days. The average rate constant of the gold nanoparticle-containing plasmonic leaf catalyzed reaction, based on n = 6 independent experiments, is shown in the figure.


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kCycle 1= (1.86 ± .138) * 10-4 sec-1

Figure 7. Reusability of the gold nanoparticle-containing leaf catalyst. A total of four recycles (cycles 2-5) were investigated after the first use (cycle 1). The reaction rate constant was normalized for a given cycle with respect to that obtained for cycle 1. The normalized rate constant of the p-nitrophenol conversion reaction is reported in the figure. Significant differences (paired t test) in rate constants were not observed for cycles 2-4 compared to cycle 1, while a statistically significant change in rate constant was observed for cycle 5 compared to the first cycle.


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AUTHOR INFORMATION Corresponding Author *To whom all correspondence must be addressed Kaushal Rege Email: [email protected] Fax: 480-727-9321 Phone: 480-727-8616 Present Addresses 501 E. Tyler Mall, ECG 303 Arizona State University Tempe, AZ 85287-6106

ACKNOWLEDGMENT This research was supported by the Defense Threat Reduction Agency (DTRA) Young Investigator Award (HDTRA1-10-1-0109) to KR. We gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University, and sincerely thank Dr. Karl Weiss for several helpful discussions and invaluable technical assistance on SEM and TEM studies. The authors acknowledge Mr. Kyle Staggs and Mr. Ming Liu for their assistance with SEM and TEM imaging. The authors also thank Mr. Taraka Sai Pavan Grandhi, a graduate student and Dr. Bhavani Miryala, post-doctoral scientist in KR’s laboratory, for assistance with confocal microscopy and FT-IR spectroscopy, respectively.

SUPPORTING INFORMATION Experimental results on Scanning Electron Microscopy Images of intact leaf venation and Nanoparticle distribution in different regions of the leaf are shown in the Supporting Information section. This material is available free of charge via the Internet at http://pubs.acs.org.


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SILVER ACS Paragon Plus Environment

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Biotemplating Plasmonic Nanoparticles Using Intact Microfluidic

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