Inkjet-Based Fabrication Process with Control over the Morphology of

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Inkjet-based fabrication process with control over the morphology of SERS active silver nanostructures Pushkaraj Joshi, and Venugopal Santhanam Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04663 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Industrial & Engineering Chemistry Research

1

Inkjet-based fabrication process with control over the morphology of SERS active silver nanostructures Pushkaraj Joshi and Venugopal Santhanam*

Department of Chemical Engineering, Indian Institute of Science, Bangalore-560012 India Tel: 91-80-2293 3113 Fax: 91-80-2360 8121 Email: [email protected]

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 ABSTRACT Morphological control of silver nanostructures is sought after due to the dependence of optical properties on size and shape. Herein, we report a facile printing process for fabricating silver nanostructured films with wire-like or particle-like morphologies on paper by merely varying the halide composition of precursor salt from potassium bromoiodide to potassium chloride. Silver was efficiently retained on the top surface of porous paper substrates by printing an excess of potassium halide salt first, leading to more conductive films at lower silver loadings. Furthermore, Raman characterization results show that silver films having nanoparticle morphology have higher SERS activity than samples with nanowire morphology, although the roughness factor of nanowire films is higher than the corresponding nanoparticulate film. Overall, these findings highlight a facile process for controlling the morphology of SERS-active silver nanostructures, which are fabricated on paper using a desktop inkjet printer.

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Keywords Percolation, Inkjet Printing, Nanoparticle, Nanowires, Silver, SERS

1. INTRODUCTION Metal nanostructures are widely investigated for their size and shape-dependent properties that manifest in the nanometer size range.1 The size and shape-dependent optical,2,3 electrical4 and catalytic5 properties of noble-metal nanostructures make them an essential component for emerging applications such as SERS (Surface Enhanced Raman Spectroscopy) substrates,6 LSPR (Localized Surface Plasmon Resonance) sensor,7 strain sensor,8 and photocatalysis.9 For exercising control



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4 over the shape and size of colloidal solutions of noble-metals several strategies like seeded growth,10–12 optimization of process conditions,13and laser irradiation14,15 have been explored. The SPR can be tuned by varying size and shape of the individual nanostructures,2,3,16 however, LSPR,17 and SERS18 properties are also dependent on the proximity and arrangement of nanostructures within a film. Our group is interested in developing simple processes to fabricate low-cost, disposable, SERS substrates for on-field diagnostics.19 SERS is a promising technique for field-based detection of a wide range of analytes.20,21 Significant research efforts are directed towards optimizing morphology and maximizing the Enhancement Factor (EF) of SERS substrates.10,16,22,23 In this context, nanoparticles, nanoplates (triangles, hexagons) and nanowires (nanorods) have been typically investigated. Two critical aspects of morphological change that affect EF values are; 1) the presence of sharp features on nanostructures that contribute to electromagnetic signal enhancement due to ‘lighting rod effect’,24 and 2) selective adsorption of analytes on different crystal planes that can lead to changes in the chemical enhancement of Raman signal.10 The confounding effects of these two phenomena on EF values has led to conflicting claims in the literature regarding the effectiveness of nanoparticles25,26 vis-à-vis nanowires/nanoplates/nanorods.10,22 Direct comparison of the results from these different studies is not feasible as they differ in the nature of dielectric environment around nanostructures, their optical arrangement, and the state of aggregation of nanostructures. All of these factors significantly affect the measured Raman signal. Thus, there is a need for experiments to compare the SERS effectiveness of various nano-metallic morphologies under an otherwise similar dielectric and optical environment. For typical on-field SERS diagnostic applications, the optimized nanostructures need to be assembled over a flexible substrate. Paper is the preferred choice of substrate as it is affordable, biodegradable and easily disposable by incineration.21 Techniques used for patterning metal


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5 nanostructures on the paper include pen on paper,27 self-assembly,28 filtration,29 dip-coating,7 screen-printing,30 inkjet printing,31–35 in-situ chemical reduction36 and physical vapor deposition.37 Amongst these, inkjet printing is most sought-after38 due to its simplicity and amenability for scaling up for high volume production. Typically, the success of inkjet printing relies on the formulation of ink that is made by processing colloidal solutions and modifying their physical properties using additives for imparting jettabilty through nozzle head. Furthermore, surface modifications like smoothening,39 imparting hydrophobicity,31 are required to avoid seeping of the ink into the underlying fibres and ensuring efficient utilization of the nanostructured ink. Recently, we had demonstrated the use of a ‘print-expose-develop’ process for fabricating SERS-active silver nanowire networks on paper using a desktop inkjet printer,19 which obviates the need for ink formulation. To address the need to fabricate various nanostructure morphologies on paper substrates and to compare their SERS effectiveness under identical dielectric environments, we systematically varied the process conditions for fabricating silver nanostructures on paper and characterized the resultant nanostructures in terms of their morphology and SERS effectiveness. In this manuscript, we first discuss the effect of increasing the halide to silver salt ratio on the retention of silver nanostructures at the top surface, thereby achieving similar film conductivities at a lower metal loading (i.e. the amount of silver printed per unit area of the paper substrate, neglecting surface roughness). Next, we present the results of the effect of varying halide composition on the formation of wire-like or particle-like morphologies of silver nanostructures on paper. Finally, we discuss the results of the comparison of SERS effectiveness of wire-like and particle-like morphologies having similar dielectric environments and metal loadings. To our knowledge, this is the first report on an inkjet-based process to form SERS-active silver nanostructures on paper with the ability to produce either wire-like or particle-like morphologies.



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6 2. EXPERIMENTAL SECTION 2.1. MATERIALS Silver nitrate, potassium bromide, potassium iodide, potassium chloride, metol, hydroquinone, sodium sulphite, anhydrous borax salt were of AR grade and purchased from SD Fine Chemicals. Raman probe molecule, malachite green oxalate (MG) (≥ 90% pure), was also procured from SD Fine Chemicals. DI water (18 MΩ-cm) used in all the experiments was from a Merck MilliQ® unit. Tissue paper – (Kimwipe® – LINTGUARD®) was used as a substrate for inkjet printing. HP (J1100) Deskjet printer with 802 cartridges was used for printing precursor salt solutions. 2.2.

METHODS

2.2.1. INKJET PRINTING AND PHOTOGRAPHIC DEVELOPMENT OF SILVER HALIDES The inkjet printing process is adapted from our previous report.19 Briefly, silver halide crystals were formed in situ on tissue paper by consecutively printing potassium halide and silver nitrate salt solutions using a desktop inkjet printer (HP J1100). The values for silver loading reported here were obtained by measuring the volume of solution dispensed by the inkjet cartridge over a known footprint area (i.e. area of the pattern generated in the software, neglecting surface roughness of the substrate), and using the concentration of the silver nitrate solution. The patterned silver halide film was then exposed to a commercially available halogen lamp (Crompton Greaves - Model # J240V, 500 W, R7S, 9500 Lumens) for 15 minutes at a distance of 50 cm. The substrates were then dipped in a standard photographic developer (D-76) for 5 minutes followed by rinsing in DI water. For silver chloride substrates, 3% KCl was added to the D-76 developer to suppress desorption of silver


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7 chloride into the developer. Finally, the substrate was dried in a laminar hood or vacuum desiccator.

Figure 1: (a) A schematic illustration of the steps involved in the fabrication process. (b) A chemical reaction scheme for the formation of silver from silver halide by photographic processing adapted from an earlier report.40 Figure 1 shows a schematic of the fabrication process, as well as a reaction scheme for the

conversion of silver halide into silver that is adapted from literature on silver halide photography.40 Briefly, photoelectrons generated within the silver halide crystal, react with silver ions at the kink sites (crystal defect sites on the surface) to form nuclei. The nuclei (latent images) that can be developed have at least four silver atoms. During development, the growth occurs by electron transfer between the reducing agent and silver ions. This step is catalysed by the nuclei acting as electron reservoir (vide infra). 2.2.2. ELECTRICAL CHARACTERIZATION The sheet resistance of the fabricated films was measured with a portable, handheld, four probe meter– R-CHEK (Model# RC2175).



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8 2.2.3. IMAGE ANALYSIS FOR SIZE DETERMINATION Field Emission Scanning Electron Microscope (FESEM) images were recorded with secondary electron detector (Inlens and SE2) on the ULTRA55 machine from ZEISS. Opensource software ‘Image J’41 was used for performing nanoparticle and nanowire size analysis. Since the silver nanoparticle samples were sparse and non-conductive, a gold coating of 5 nm was used to avoid charging issues for obtaining the high magnification images of nanoparticle samples shown in fig 7. All other FESEM images were obtained as is, i.e. without coating, as there were no issues with charging during imaging of those samples. 2.2.4. SERS CHARACTERIZATION AND SIGNAL ANALYSIS Raman signal was recorded using a Horiba Jobin-Yvon LabRAM™-HR Raman instrument equipped with 532 nm and 785 nm laser sources, and liquid nitrogen cooled charge coupled device (CCD) acted as the detector. Raman signals were collected using the D2/D3/D4 optical filter on incident laser source (532 nm and 785 nm) with 5 s integration time and 3x averaging. The optical assembly comprised of a 0.9 NA 100X objective. Raman band intensity at 520.5 cm−1 of a reference silicon wafer was used to calibrate the spectrometer and normalize the intensities collected over time. Spectra were recorded from at least 27 random locations on the sample, which were more than 100 µm apart and covered all the regions of the SERS substrate. The SERS signals were processed with COBRA software42 for background correction and smoothening on MATLAB R2014b (The MathWorks, Inc., Natick, MA, USA). Data analysis was accomplished using Origin 9.0 software (OriginLab Corporation, Northampton, MA, USA). The SERS intensities corresponding to the desired peaks were integrated over their width, and the average value with


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9 error bars corresponding to ± 95% confidence interval are reported for comparison of SERS signal strength. 3. RESULTS AND DISCUSSION 3.1. EFFECT OF PRINTING EXCESS HALIDE The fabrication process involves the alternate printing of halide precursor and silver nitrate salt solutions on paper as reported earlier.19 The porosity of the paper substrate would lead to penetration of salt solutions into fibres below the paper surface and lead to sub-optimal utilization of silver for applications requiring conductive nanostructured films. To efficiently retain the printed silver on the paper surface, we hypothesized that if an excess of halide salt was printed first and was then followed by printing of silver salt, a higher amount of silver salt would react with halide ions present on the top surface and remain on the surface. Thereby, leading to higher coverage of silver halide on the surface fibres and consequently, higher coverage of silver nanowires on the surface after development. Such retention of metal ions on the top surface was evident in an earlier report,43 wherein printing excessive amounts of reducing agent followed by a metal salt improved the electrical conductivity of the silver film. Figure 2 (a)-(c) shows a steady increase in the surface coverage of silver halide due to an increase in the molar ratio of halide to silver salt from 2/1 to 8/1 while maintaining the same silver loading of 0.135 mg/cm2. These silver bromo-iodide films upon development show a corresponding increase in the silver nanowire coverage on the paper surface (Figure 2 (d) – (f)). Further increase in the halide to silver salt ratio to 10/1 led to an excessive delay in development time and therefore was not investigated further. The sheet resistance measurements, Figure 3(a), show that using an excess of halide leads to lower sheet resistances (enhanced conductance) at a given silver loading. For instance, the sheet resistance obtained at



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10 0.196 mg/cm2 is 5900, 336, and 320 Ω/□ corresponding to halide to silver molar ratio of 2/1,4/1 and 8/1, respectively. The variation of the sheet resistance of the silver nanowire films as a function of silver loading agrees reasonably well with the percolation network model (eq. 1) for sheet resistance (Figure 3(b)).

(1)

Where, threshold,

– sheet resistance,

– proportionality constant,

– silver loading beyond the threshold,

– critical silver loading at the

– scaling exponent. The scaling exponent

depends on the dimensionality of the system.

Table.1: The critical silver loading and scaling exponent for different halide to silver molar ratio. Halide to silver molar ratio (X/Ag)

Critical silver loading (mg/cm2)

Scaling exponent fit

2/1

0.15

1.94

4/1

0.078

1.96

8/1

0.039

1.99


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11

Figure 2: Representative FESEM images of inkjet printed silver bromide-iodide films (a-c) and corresponding silver nanostructures after development (d-f) fabricated by varying halide to silver salt (X/Ag) concentration ratio while maintaining a silver loading of 0.135 mg/cm2 on Kimwipe tissue paper. (a,d) X/Ag – 2/1, (b,e) X/Ag – 4/1, (c,f) X/Ag – 8/1. The computed surface coverage values of silver halide are also shown in a-c.



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12 Table 1 enumerates the critical silver loading and the scaling exponent obtained by fitting the percolation model to the sheet resistance of the films fabricated using different halide to silver salt ratios. The decreasing values of the critical silver loading required for percolation and a concomitant increase in surface area coverage of silver halide film (see Figure 2 a-c) with an increase of halide content further corroborates the observation based on FESEM images regarding the effective retaining of silver on the top surface of the paper. The scaling exponent values for all the samples agree closely with 1.9, a value expected for a 3D percolating network.44


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13

Figure 3: (a) Variation of sheet resistance as a function of silver loading for different halide to silver salt ratios (X/Ag) – 2/1, 4/1 and 8/1. The lines are an aid to the eye. (b) A plot of the sheet resistance as a function of reduced silver nitrate loading (i.e. loading normalized with respect to the critical value). The points represent the measured data while the line represents a model fit (based on a nonlinear curve fit) to the equation based on percolation theory (eq. 1).



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14 3.2 MORPHOLOGY CONTROL BY VARYING THE HALIDE COMPOSITION

Figure 4: Representative FESEM images of silver nanostructured films formed by development of (a) silver bromide-iodide film, (b) silver chloride film and (c) silver chloride-iodide film. The halide to silver molar ratio of 2/1 was maintained with an overall silver loading of 0.5 mg/cm2. The insets show corresponding XRD patterns of the developed silver samples. (d) Photographically developed silver chloride film with the silver loading of 1.5 mg/cm2 showing the presence of larger polyhedral silver nanoparticles. Next, the effect of silver halide composition on the morphology of developed silver was investigated. For these experiments, the halide to silver molar ratio was maintained constant at a value of 2. The morphology of developed silver could be varied from nanowire to nanoparticles by


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15 merely changing the halide composition from bromide-iodide (Br/I::95/5) to chloride (Figure 4a,b). Additionally, when some amount of iodide was added to chloride (Cl/I::95:5), the developed silver exhibited a mixed morphology with the presence of both nanowires and nanoparticles (Figure 4c). The corresponding insets in Figure 4 show XRD pattern of developed silver obtained from corresponding silver bromo-iodide, silver chloride or silver chloro-iodide crystals. The presence of (111), (200) and (220) peaks corresponding to FCC silver (JCPDS- file no 04-0783) confirm the formation of silver nanostructures in all the cases. The photochemistry of silver halide photography process is examined for understanding the resultant morphology. Photoreduction of silver halide into silver nuclei is key to photography process, and the silver halides differ in their ability to absorb incident photons. For instance, silver chloride absorbs mainly in the ultraviolet region and absorption by silver bromide extends to the range of blue light. Also, the addition of small amounts of iodide is shown to extend the photoabsorption range towards visible part of the spectrum,45 thereby assisting in the formation of silver nuclei (referred to as “Latent images”) on silver halide particles under visible light. Also, the presence of iodide is known to increase the lattice constant of the silver halide crystal and increase the number of defect sites (kinks) due to lattice mismatch.40 The presence of such kinks on the crystal surface facilitates the formation of silver nuclei upon photo exposure. Such silver nuclei also trap photoelectrons generated inside the crystal, thereby, increasing the production of silver ions in the crystal.46



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16

Figure 5: Schematic illustrating the development process for formation of silver nanowire filaments from photoexposed silver bromoiodide films and silver nanoparticles from silver chloride film, The black outline represents the disintegrating silver halide grain from which the silver ions are being reduced while the halide ions are being solvated. The magnified view of the boxes near the growth interfaces show cartoons of the proposed growth mechanisms. Filamentous nanowires grow by accretion of mobile silver ions that are reduced at the base of the latent image,46 while nanoparticulate morphology evolves when the growing metal front encounters silver ions in the lattice. In both cases, the growing metal nanostructure acts as an electron reservoir in which the redox pair donates electrons and from which the silver ions gain electrons. Figure 5 shows schematic illustrations of the processes involved in the development of silver halide grains. The formation of nanowire networks occurs when the mobility of silver ions within the halide crystal is high, and so, the silver ions accrete at the base of the growing nuclei and are then reduced by electrons donated to the silver nuclei by the developing agent. The asymmetric transport of silver ions to one side of the growing nuclei leads to the formation of nanowires.40 If there are multiple nucleation sites on a halide crystal surface, then these multiple wires are interconnected either by fusion of wires growing within a halide crystal or by the fusion of wires


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17 growing out from neighbouring halide crystals.47 The growth direction of nanowires, either into or protruding out of the crystal will depend on the local stress gradients generated by lattice defects and by the release of bromide ions from the lattice into solution. Thus, introducing larger iodide species into the halide grain should facilitate the formation of nanowires, as it generates more lattice defects that increase silver ion mobility. To verify the role of iodide in the formation of a dense network of nanowires, we printed a silver bromide (i.e. without iodide) film (Figure 6a) and further processed it to form silver nanostructures (Figure 6b,d). This resulted in very thick (nominal diameter ~ 147 nm) and sparse nanowires, as would be expected for a lower number of latent images within a bromide crystal. In contrast, upon the inclusion of 5% iodide salt during printing, the photographically developed silver (Figure 6c) forms a uniform network of thin nanowires (nominal diameter ~ 37 nm). Due to the formation of uniform and thin nanowires that bridge the adjacent halide crystals, the film is conductive with a sheet resistance ~ 274 Ω/□, whereas the sample fabricated without iodide is non-conductive. In the case of silver chloride crystals, the lattice constant is smaller and also the number of latent images formed is lesser, due to lower absorptivity in comparison to the bromo-iodide samples. The smaller lattice size restricts the mobility of silver ions towards the surface leading to the development of the chloride crystals by inward diffusion of the electrons donated to the surface kink sites by the developing agent. We hypothesize that the transport of electrons from the surface nucleation sites to the growing metal front, without any preffered direction, leads to the formation of polyhedral silver nanoparticles after development. The size of the developed polyhedral silver nanoparticles depends on the initial size of the silver chloride crystals, which in turn is dependent on the amount of silver salt printed over a unit area (Figure 4(d)). Thus, control over silver nanostructure morphology and size is easily achieved by controlling the halide composition and the silver loading used in the printing process.



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Figure 6: Representative FESEM images of (a) Inkjet printed silver bromide film with a silver loading of 0.25 mg/cm2, and halide to silver molar ratio of 2/1. (b) Photographically developed silver bromide film and it's (d) magnified image. (c) A photographically developed silver bromideiodide film with a silver loading of 0.25 mg/cm2, and halide to silver molar ratio of 2/1. 3.2. EFFECT OF MORPHOLOGY ON SERS ACTIVITY The control over silver nanostructure morphology as well as their sizes within films fabricated by inkjet printing enables us to study their SERS activity under similar optical, dielectric and silver loading conditions. Malachite green was used as a non-resonant SERS probe molecule. Figure 7(a) compares the SERS spectrum of MG obtained by dipping substrates with a silver loading of 0.067 mg/cm2 exhibiting nanoparticulate and nanowire morphology in a 1mM solution of MG. The characteristic peaks of MG27 namely 1618 cm−1 (ring C–C stretching), 1397 cm−1 (N-phenyl stretching), 1173 cm −1 (ring C–H in-plane bending) and 917 cm−1 (C–H out-of-plane bending) are clearly observed in the spectrum. The comparison of spectra indicates that peak positions and relative intensities are independent of the morphology. This signifies that the adsorption configurations of the probe molecule are comparable for both morphologies and so their SERS activity can be directly compared. Figure 7(b) shows that nanoparticulate morphology has a


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19 significantly higher SERS signal than nanowire morphology for a given silver loading. The ratio of the SERS signal from samples having nanoparticle morphology to that of samples having nanowire morphology is ~10 at lower values of silver loading and then attains a value of ~ 2 at much higher silver loading. Figure 7: SERS signal comparison of 1mM malachite green from substrates with nanoparticle and

nanowire morphology obtained with 0.067 mg/cm2 silver loading, and halide to silver molar ratio of 2/1. (b) SERS signal evaluation from substrates with nanoparticle and nanowire morphology using 1173 cm-1 vibration. Acquisition parameters – 532 nm laser, 2.5 µW and 5 s integration time with 3x averaging.



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20 The trends in SERS activity as a function of silver loading and morphology could arise due to either change in electromagnetic enhancement effect or chemical enhancement effect. As the XRD patterns of samples having either nanowire or nanoparticle morphology (Figure 4 insets) indicate the presence of (111), (200) and (220) facets in similar proportion, the role of chemical enhancement due to different adsorption geometries can be ruled out. This is also consistent with the fact that the spectral signature of the probe molecule did not change with nanostructure morphology (Figure 7a). Next, to ascertain if changes in roughness values (actual surface area per unit geometric footprint area) can account for the observed changes in SERS activity, we obtained representative FESEM images of all the samples (Fig. 8 & SI Fig. S1 to S6). Nominal values for the nanoparticle diameter as well as nanowire diameters were determined after image analysis (see SI Fig S7 to S8). These values were used to compute a roughness value as described in our earlier report19 (example calculation details are presented in SI). The roughness factor for nanowire samples is consistently higher in comparison to nanoparticle sample for all the silver loadings studied (See Table S1). However, the SERS signal from nanoparticle samples is consistently higher than that of nanowires. Thus, the availability of high surface area for adsorption of analyte molecules through higher roughness factor does not correlate with the variations in SERS activity. This suggests that the effect of surface area is not significant enough to offset the effect of morphology on SERS activity. Consequently, the electromagnetic enhancement must be the dominating factor contributing towards the observed trends of SERS activity as a function of silver loading.


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21

Figure 8: Representative FESEM images of photographically developed silver halide films with nanoparticle and nanowire morphology for samples with various silver loading. The nanoparticlebased substrates were coated with 5 nm gold film to avoid localized charging during imaging. Magnified views are available in SI (Fig. S1-S6)



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22 The presence of faceted nanoparticles with several sharp edges is a critical factor in determining Raman enhancement. Previous reports10,22 also indicate that faceted nanoparticles with sharp edges display higher SERS signal. In our case, the presence of a combination of faceted nanoparticles like cubes, plates (triangles, hexagons) and polygons with sharp edges may account for the higher SERS signals from the nanoparticulate samples. Additionally, FDTD evaluations48 of electrical field enhancement around dimers of nanorods have revealed that there is a significant red shift of the resonant wavelength into infrared regions when the nanorods are coupled conductively. Given the conductive nature of our nanowire samples, this could be another plausible reason for the lower SERS enhancements in comparison to nanoparticle samples. At the lower loadings, the sizes of the nanowires are too small to scatter electromagnetic radiation efficiently. Furthermore, the SERS signal undergoes a saturation for both nanoparticle and nanowire morphologies at higher silver loadings. It is observed that the sizes of the nanostructures do not change significantly over this range of silver loading and although the density of hotspots should increase with increased loading, the inaccessibility of the hotspots due to occlusion and / fusion of nanostructures could be a cause for signal saturation. Additionally, when we compared the SERS signal using a 785 nm laser source, the effect of morphology on SERS activity is similar (Figure 9a). This indicates that the effect of morphology on SERS activity is independent of the laser illumination over this range of wavelengths. Finally, we quantified the EF values of the two different morphologies by comparing their signals with that of a bulk sample of the probe molecule (Figure 9b). The average SERS substrate enhancement factor based on geometric footprint area was calculated using the following equation.

(2)


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23 NSERS represents the number of molecules adsorbed on the substrate giving rise to SERS signal, and Nbulk, the number of molecules in the bulk sample that fall within effective signal collection volume. The measurement of the effective depth of the signal collection from bulk samples was carried out as described earlier.19 Nbulk was estimated assuming that all the signal was collected from a cylindrical volume with a cross-sectional area corresponding to the laser spot size and effective depth as determined above. NSERS was calculated assuming complete monolayer coverage of probe molecules over the geometric footprint area under laser illumination. Thus, the enhancement factor for nanoparticle-based substrate was determined to be ~ 5.7

x

105 based on

1173 cm-1 vibration and 6.1 x 105 based on 1618 cm-1 vibration. Whereas, the enhancement factor for nanowire-based substrate was determined to be ~ 3.1 x 105 based on 1173 cm-1 vibration and 3.7 x 10

5

based on 1618 cm-1 vibration (See SI for more details). These EF values are comparable with

the values reported in literature27 for paper-based SERS substrate.



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24

Figure 9: (a) SERS signal comparison of substrates with nanoparticle and nanowire morphology using 1173 cm-1 vibration under 532 and 785 nm laser illumination with the constant silver loading of 0.067 mg/cm2, and halide to silver molar ratio of 2/1. Acquisition parameters for 532 nm laser 2.5 µW and 5 s integration time with 3x averaging; 785 nm laser - 0.2 mW laser power, 5 s integration time and 3x averaging. (b) Evaluation of average SERS substrate enhancement factor by comparison of SERS signal from the substrate with nanoparticle morphology and nanowire morphology, with the silver loading of 0.5 mg/cm2 and halide to silver molar ratio of 2/1, against bulk Raman signal under 532 nm laser illumination. SERS signal was obtained with laser power – 2.5 µW and 5 s integration time with 3x averaging. The bulk Raman signal was obtained with a laser power of 25 µW and 5 s integration time with 3x averaging.


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25 4. CONCLUSION In conclusion, we have demonstrated the ability of the ‘print-expose-develop’ process47 to fabricate nanostructured films having morphologies varying from nanoparticles to nanowires and combinations thereof, by merely varying the halide composition. Also, the use of excess halide to efficiently retain the silver on the surface of a paper substrate has been established. This enabled the production of highly conductive patterns on paper using a silver loading of only 200 µg/cm2 of geometric print area. The SERS activity of nanoparticle samples is found to be higher than nanowire samples, under otherwise identical conditions. The analysis of the trend of SERS activity as a function of silver loading shows that the electromagnetic enhancement due to the presence of sharp features is responsible for increased SERS signal in the case of nanoparticle samples. These trends are also found to be valid for illumination using an IR source. Overall, we have demonstrated a facile process for fabricating SERS active silver nanostructured films with controlled morphologies on a paper substrate, and this work will enable the fabrication of low-cost, disposable SERS based sensors for a wide range of applications.

ASSOCIATED CONTENT The supporting information contains magnified views of FESEM images of nanostructures at various silver loadings, the size distribution details of nanoparticles and nanowires under different silver loading obtained by analyzing FESEM images using ImageJ, estimation of roughness factor for nanoparticles and nanowires under different silver loadings, Details of SERS EF computations.



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26 AUTHOR INFORMATION CORRESPONDING AUTHOR * [email protected] AUTHOR CONTRIBUTIONS The manuscript was written through contributions from both the authors. The authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT SERB is acknowledged for funding. CeNSE, IISc is acknowledged for providing FESEM, Raman facility. MHRD is acknowledged for a graduate fellowship. REFERENCES (1)

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