TEMPO-Oxidized Nanocellulose Fiber-Directed Stable Aqueous

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TEMPO-Oxidized Nanocellulose Fiber-Directed Stable Aqueous Suspension of Plasmonic Flower-Like Silver Nanoconstruct for Ultra-Trace Detection of Analytes Kallayi Nabeela, Reny Thankam Thomas, Jyothi B Nair, Kaustabh Kumar Maiti, Krishna Gopa Kumar Warrier, and Saju Pillai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07295 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

TEMPO-Oxidized

Nanocellulose

Fiber-Directed

Stable Aqueous Suspension of Plasmonic FlowerLike Silver Nanoconstruct for Ultra-Trace Detection of Analytes Kallayi Nabeela,†,‡ Reny Thankam Thomas,† Jyothi B. Nair,‡,§ Kaustabh Kumar Maiti,‡,§ Krishna Gopa Kumar Warrier† and Saju Pillai*,†,‡ †Functional Materials, Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram, Kerala-695 019, India ‡Academy of Scientific and Innovative Research (AcSIR), New Delhi-110 001, India §Organic Chemistry, Chemical Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram, Kerala-695 019, India KEYWORDS: TEMPO-oxidized nanocellulose fiber, silver morphologies, anisotropic nanostructures, Surface Enhanced Raman Scattering, green synthesis.

ABSTRACT: Synthesis of shape-tuned silver (Ag) nanostructures with high plasmon characteristics has drawn significant importance in in-vitro diagnostic applications. Herein, we report a simple aqueous synthetic route using TEMPO-oxidized nanocellulose fibers (T-NCF)

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and trisodium citrate (TSC) that results in anisotropically grown flower-like Ag nanoconstruct (AgNF). A detailed investigation of the concentration and sequence of the addition of reactants on the formation of these anisotropic Ag structure is presented. Our experimental results show that the mechanism underlying the formation of AgNF is facilitated by the synergistic action of T-NCF and TSC on the directional growth of Ag nuclei during the primary stage which later develop into a flower-like structure by ripening of larger particles consuming smaller Ag particles. As a result the final structure comprises of flower-like morphology over which several smaller Ag particles (of size 4 months) by the efficient capping action of T-NCF. Further, as-synthesized nanoconstruct shows excellent surface enhanced Raman scattering (SERS) activity which enables ultrasensitive detection of paraaminothiophenol (pATP) with a concentration down to 10 aM (10-17 M) in a reproducible way. This bio-supported synthesis of stable aqueous colloids of AgNF may find potential applications as biomedical sensing platform for the trace level detection of analyte molecules.

1. INTRODUCTION Synthesis of anisotropic morphologies of size and shape tuned noble metal nanostructures with enhanced surface plasmon resonance are promising for the design of excellent platforms for catalytic,1-2 sensing,3 biological labeling and imaging4-10 applications. Surface enhanced Raman spectroscopy (SERS) based sensing is considered as the most powerful non-invasive method that allows the trace level detection of analytes.5 When plasmonic nanoparticles like Ag, Au, Cu, etc. are in close proximity, the enhancement of the electromagnetic field show a rapid increase due to coupling of individual localized surface plasmon resonances (LSPRs), generating hot spots


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responsible for sensitive detection of analyte molecules. Indeed, it is well known that branched nanostructures are one type of attractive morphology where folds, apexes and joints of the nanoconstruct are expected to provide more favorable centers for hot spots than nanosphere or nanowire morphologies of the same metals.6 However, in the case of noble metals, acquiring highly anisotropic morphologies are complicated as they are prone to achieve highly symmetric face-centered cubic (fcc) crystal structure. Although several reports exist regarding synthesis of noble metal nanocrystals of symmetric shapes like spherical, rod, prism, cube and sheet, synthesis of highly anisotropic Ag morphologies demonstrating ultrasensitive detection of analyte molecules are limited. The choice of solution-phase preparative methods for the aforementioned morphologies depends on a number of experimental parameters that control the nucleation and growth processes during synthesis. In addition to reducing agents, the most widely used strategy involves the use of an appropriate capping agent that preferably adsorb onto specific crystal planes and suppress the growth in particular directions resulting in kinetically favored morphologies.7 For example, Archer and coworkers reported spontaneous growth of coral-like highly branched Ag morphology from bulbous seed upon reduction using two reducing agents, TSC and L-ascorbic acid, of different reactivity, where the anisotropic growth occurred due to specific adsorption of citrate on Ag (111) plane.8 To this end, several researchers have reported the synthesis of noble metal with challenging anisotropic morphologies like nanostars,9 nanoflowers,4,

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nanodendrites,16-19 etc. However,

many such methods follow environmentally malignant synthesis route where corrosive shapedirecting agents like halides or toxic solvents are used. In a synthetic approach by Cathcart et al., multipod faceted morphologies of Au and Ag were prepared via anisotropic etching which involve the use of corrosive halide.19 Earlier, Raveendran et al., reported green synthesis of Ag


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nanoparticles by the use of environmentally benign starch as reducing agent as well as stabilizing agent.20 Following that, green synthesis of Ag and Au nanostructures using various biological entities like plant extracts,21-22 vitamins,23-24 sugars25-27 amino acids28-29 and proteins30-31 for reduction and stabilization, have received great attention. Recently nanocellulose fibers (NCF), a unique class of sustainable material, has been demonstrated as a platform for nanoparticle synthesis.2,

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In addition to high specific surface area, excellent colloidal stability of

negatively charged NCF make them suitable for surface-mediated nanoparticle synthesis.37-38 There exist a few reports dealing with nanocellulose for the synthesis of Ag nanostructures.33, 3637, 39

However, the effect of surface functionalized nanocellulose on anisotropic Ag morphologies

have not been widely explored. Stable aqueous suspension of Ag nanostructures can be obtained by tuning the surface chemistry of nanocellulose.37,38 The carboxyl functionalized cellulose nanocrystals (CNCs) produced by TEMPO-oxidation yields higher anionic group concentration compared to sulfate groups and has been used to control the size distribution of AgNPs.38,40 Furthermore, TEMPO-oxidized NCF formation is considered as the most cost-effective approach to extract nanocellulose from plant origin.40-41 Herein, we utilize TEMPO-oxidized NCF obtained from banana fiber (a natural fiber) for synthesizing aqueous colloids of flower-like Ag nanoconstruct (AgNF) with excellent colloidal stability. A time-dependent study on growth and investigation of the role of each reactant in formation of these anisotropic morphologies is performed. Further, the aqueous colloidal nanoconstructs are utilized for ultra-trace detection of pATP at concentration lower than 10 aM. The enhanced SERS activity is obtained without the use of any aggregating agents like NaCl or KCl, which highlights the significance of the Ag nanoconstructs formed as efficient Raman sensing platforms (SERS substrate).


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2. EXPERIMENTAL SECTION 2.1. Materials. Bio-extracted banana pseudo-stem fibers (BPSF) were used for nanocellulose fibers extraction. Silver nitrate (AgNO3), trisodium citrate tribasic dihydrate (C6H5Na3O7. 2H2O), sodium borohydride (NaBH4), TEMPO (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl; C9H18NO), sodium chlorite (NaClO2), sodium bromide (NaBr), 15% solution of sodium hypochlorite (NaClO), sodium hydroxide (NaOH) and potassium hydroxide (KOH), methylene blue (C16H18ClN3S. 3H2O) were purchased from Sigma-Aldrich, pATP (para-aminothiophenol; C6H7NO) purchased from Alfa Aesar and used as received. All standard solutions were prepared using ultrapure deionized water of resistivity 18.2 MΩ.cm at 25 oC (Milli-Q purifier system, Merck, Germany). 2.2. Extraction of nanocellulose. Removal of wax and non-cellulosic contents from raw BPSF were done using the protocol described elsewhere42-43 with minor modifications. Briefly, bio-extracted banana fibers were mercerized with 15 % NaOH solution for 2 h at room temperature, bleached three times by refluxing with 1.7 wt% of NaClO2 acidified with glacial acetic acid (pH 4). Resultant bleached fibers were treated with KOH solution to remove trace amount of non-cellulosic contents if any exist. Holocellulose thus obtained was filtered, washed thoroughly, dried and desiccated. 2.3. Preparation of TEMPO-oxidized NCF. Nanocellulose fibers were extracted from holocellulose by TEMPO-mediated oxidation followed by ultrasonication.44 The holocellulose fibers were subjected to TEMPO-oxidation adopting the protocol described by Saito40-41 with minor modifications. Briefly, 0.016 g (0.1 mM/g of cellulose) of TEMPO and 0.16 g (1.6 mM/g of cellulose) of NaBr were taken in a two-neck RB flask previously weighed with 1g of


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desiccated holocellulose, dissolved in DI water. 10 mL of NaClO was added drop-wise into the mixture while stirring at room temperature maintaining the fiber to liquor ratio 1:200 by adding required amount of DI water (The pH was adjusted to 10-11 throughout the oxidation process using 0.5 M NaOH solution). As the oxidation reaches completion, the pH remains unchanged. The whole system was then transferred into a sonication bath for further disintegration of oxidized fibers by ultrasonication. The sonication temperature was kept constant at 25 °C throughout the reaction. About 5 mL of ethanol was then added to the highly dispersed suspension to avoid further oxidation. It was centrifuged and washed several times to remove excess reagents, freeze-dried and desiccated for further use. 2.4. T-NCF reduced AgNPs. The Ag+ ion reduction was carried out using T-NCF alone. Briefly, 100 mL of freshly prepared AgNO3 (0.25 mM) solution containing 0.25 wt% of well dispersed T-NCF (with the help of sonication) was heated at its boiling temperature for 1 h. The resultant AgNP solution was cooled at ambient temperature and stored under dark condition. 2.5. Single step AgNP synthesis in T-NCF/TSC system. A single step AgNP synthesis was carried out combining the effects of TSC and T-NCF. About 0.25 wt% of T-NCF was weighed into a RB flask already containing 100 mL of 0.25 mM AgNO3 and dispersed well by sonication. This mixture was heated in an oil bath. When boiling commenced, 1 mL of 1 wt% of TSC was added drop-wise into the mixture and heated further for 60 minutes. The AgNP solution obtained was cooled at room temperature and stored in dark. Pseudo-spherical AgNPs were synthesized adopting standard Turkevich method.45 For the purpose of comparison, TSC was used as reducing and capping agent keeping all other reaction parameters constant.


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2.6. Synthesis of flower-like Ag morphology. The flower-like Ag morphology was obtained by sequential addition of TSC to Ag+/T-NCF medium. Briefly, a stock solution of 1 mM AgNO3 solution was prepared first. 1 wt% of freeze-dried T-NCF was well dispersed in 25 mL of 1 mM AgNO3 and heated in an oil bath. To this solution, 0.5 mL of 1 wt% of TSC was added at its boiling point. After 5 minutes, the mixture was diluted to 100 mL by adding DI water with gentle stirring so that effective Ag+ ion and T-NCF concentrations are maintained at 0.25 mM and 0.25 wt%, respectively. Further, 0.5 mL of 1 wt% TSC was added and stirring continued for 60 minutes to get the flower-like morphology. For a more detailed study of Ag nanoparticle growth, 1 mL of sample was collected directly from reaction mixtures at different growth stages. The samples were cooled immediately after sampling to arrest further growth and stored in dark. The samples were analyzed using TEM and UV-Vis spectroscopy. The role of the precursor concentrations were investigated by varying concentrations of AgNO3 (0.15, 0.25 and 0.5 mM) and T-NCF (0.5, 0.25, 0.12 and 0.06 wt% ) keeping all the other parameters constant. 2.7. Characterizations. The size and morphology of TEMPO-oxidized NCF and Ag particles were observed using microscopic techniques. Scanning electron microscopic imaging and energy dispersive X-ray (EDX) analysis were performed using Zeiss EVO 18 cryo SEM with an accelerating voltage 15 kV. Transmission electron microscopic (TEM) images and corresponding selected area electron diffraction (SAED) patterns were taken on FEI Tecnai 30 G2S-TWIN-TEM operated at an accelerating voltage of 300 kV. Atomic force microscopy (AFM) imaging was performed under dry conditions at room temperature (22 ±2 oC) using MultiMode 8 AFM equipped with NanoScope V controller (Bruker, Santa Barbara, CA, USA). Si cantilevers (NSG 01, NT-MDT) with a typical radius of curvature of approximately 10 nm were used. The force constants of AFM probe in the range of 2.5-10 N/m and with resonance


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frequency in the range of 120-180 kHz. The scan rate used was 1 Hz. The FT-IR spectra measured with a Perkin-Elmer Series Spectrum Two FT-IR spectrometer over the wavenumber range 4000-500 cm-1. The sample was directly mixed and pelletized with KBr. Wide angle X-ray scattering measurements were carried out for T-NCF on XEUSS SAXS/WAXS system with Cu Kα radiation (λ = 1.54 Å) using Genisxmicro source from Xenocs operated at 50 kV and Powder X-ray diffraction pattern of AgNF sample was taken using PANalytical EMPYREAN instrument equipped with reference radiation of Cu Kα (λ = 1.54 Å) at an operating voltage of 45 kV. The optical absorption features of Ag colloids in the UV-Vis range of 200 to 700 nm wavelength were measured using a spectrophotometer (SHIMADZU UV-2401PC, Shimadzu, Japan) employing a 1 cm path length quartz cell at room temperature. Each solution was diluted three times prior measurements. Ζeta potential measurements were done using Nano ZS Malvern instrument. Elemental composition of AgNF colloid was obtained using ICP-MS, Thermo scientific ICAP Qc. 2.8. SERS measurements. SERS measurements was performed using WI-Tec Raman microscope (WI-Tec, Inc., Germany, alpha 300R) with a laser beam directed to the sample through 20× objective with 600 g/mm grating and a Peltier cooled CCD detector. Samples were excited with a 633 nm wavelength laser and Stokes shifted Raman spectra were collected in the range of 0-3000 cm−1 with 1 cm−1 resolution and an integration time of 2 s and 10 accumulations. WITec Project plus (v 4.1) software package was used for data interpretation. The Raman samples were prepared by mixing different concentrations of probe molecules with Ag colloid in the ratio 1:3 (v/v). The samples were incubated at least for 1 h at room temperature prior to Raman analysis. For liquid phase analyte detection, 10 µL of the sample was loaded on to a cleaned glass slide (sonicated in toluene and ethanol sequentially followed by UV-ozone


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treatment for 15 min). In solid phase detection, 10 µL of the sample was drop casted on to cleaned glass slides and dried under vacuum at ambient temperature. SERS spectra for solid samples were taken from at least six different locations. All experiments were done in triplicates. 3. RESULTS AND DISCUSSION 3.1. Characterization of TEMPO-oxidized NCF. Nanocellulose fibers of 13±3 nm width and a few hundreds nanometer length were prepared as described in the experimental section. The fiber morphology was analyzed by AFM, SEM and TEM (Figure S1, Supporting Information). The regioselective conversion of C-6 primary hydroxyl groups of cellulose into carboxylic groups by TEMPO-oxidation was confirmed from FT-IR spectra, indicated by the sharp peak at 1616 cm-1 (Figure S2a, Supporting Information) which is in agreement with reported data.41 The introduction of carboxyl functionality on nanocellulose resulted in a highly stable aqueous suspension due to electrostatic repulsive force between anionic nanofibrillated cellulose.46 This was supported by the zeta potential value of T-NCF suspension (-52.2 mV) which is better than the acid hydrolyzed one (-38.2 mV).47 Unlike acid hydrolysis, TEMPO-oxidation of nanocellulose fiber does not affect the crystallinity as evident from the XRD analysis (Figure S2b, Supporting Information). Additionally, TEMPO-oxidation results in nanocellulose fibers with Na+ ions anchored carboxylate terminals that triggered the growth of Ag clusters and form stable aqueous suspension.48 3.2. T-NCF mediated synthesis of AgNF nanoconstruct. Nanocellulose is a green alternative for the synthesis of noble metal nanocrystals due to its reducing ability and excellent capping actions.48-49 In order to obtain highly anisotropic flower-like Ag morphologies, a suitably tuned synthetic methodology has been employed using T-NCF and TSC. Figure 1a shows a


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representative bright field TEM image of three-dimensional Ag morphology with a center core underneath from which several petal-like growths protruded radially outward analogous to a flower. The size of a typical flower-like morphology was estimated to be 300-600 nm range with 5-7 primary branches (petals). The dimensions of primary branches were 200-400 nm in length and 75-200 nm in width. Furthermore, a close examination of petal-like morphology revealed the adherence of more Ag nanoclusters on them (Figure 1b) which in turn induces the growth of secondary branch (Figure 1c). As manifested from the TEM images, the aqueous colloidal solution also contained several smaller nanoparticles of sizes less than 10 nm supported over TNCF. The SAED pattern of corresponding AgNF shown in Figure 1d supported fcc lattice of Ag with polycrystalline nature. The marked spots can be indexed to (111), (220) and (222) planes of Ag fcc lattice (with inter planar distances of 2.35, 1.44 and 1.18 Å, respectively; JCPDS File No. 04-0783). The indistinct nature of SAED spots might be due to the T-NCF capping of formed AgNF morphologies. Further, an overview of AgNF morphologies over a large area is given in Figure S3 (Supporting Information). A highly branched flower-like Ag morphology of size in micrometer range (Figure 1e) was formed when a high concentration of Ag+ ions is present in the medium (see section 3.4). The corresponding SEM image over large area is given in Figure S4 (Supporting Information). Additionally, elemental mapping (Figures 1f and 1g) and EDX spectrum (Figure 1h) of a highly branched AgNF morphology have been performed to confirm the presence of elemental Ag. The total yield of Ag nanostructures was estimated as 74.9 % from ICP-MS analysis.


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Figure 1. Representative TEM images of (a) flower-like Ag morphology, (b) magnified image showing petal-like morphology with adhered Ag nanoclusters, (c) AgNF showing growth of secondary branching of petal-like morphology, (d) corresponding SAED revealing fcc lattices and polycrystalline nature of flower-like morphology, (e) highly branched AgNF morphology obtained at higher concentrations of Ag+ ions in the medium, (f) corresponding SEM image with (g) EDX-elemental mapping and (h) EDX elemental spectrum. 3.3. Effect of T-NCF and TSC on AgNF morphology formation. As a control experiment, standard Turkevich method45 was adopted where a moderately stable aqueous Ag suspension was obtained using the reducing and capping action of TSC. Here, pseudo-spherical Ag nanoparticles with high polydispersity (sizes range from 50-150 nm) were obtained (Figure 2a). This suggests that we have less control over Ag nanoparticle formation due to high reactivity of


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Ag+ ions and dual action of TSC, which complicates the selection of their optimal concentration.50 Role of T-NCF in shape control and stabilization of Ag morphology was studied utilizing its reducing action in the absence of other reducing agents. It was observed that several smaller Ag clusters of sizes

TEMPO-Oxidized Nanocellulose Fiber-Directed Stable Aqueous

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