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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Wide Range Morphological Transition of Silver Nanoprism by Selective Interaction with As(III): Tuning-Detuning of Surface Plasmon Offers to Decode the Mechanism Sudeshna Das Chakraborty, Subrata Mondal, Biswarup Satpati, Uttam Pal, Sandip Kumar De, Maireyee Bhattacharya, Sarmistha Ray, and Dulal Senapati J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10799 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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The Journal of Physical Chemistry

Wide Range Morphological Transition of Silver Nanoprism by Selective Interaction with As(III): Tuning-Detuning of Surface Plasmon Offers to Decode the Mechanism Sudeshna Das Chakraborty‡, Subrata Mondal‡†, Biswarup Satpati§, Uttam Pal‡, Sandip Kumar De‡, Maireyee Bhattacharya‡, Sarmistha Ray‡, Dulal Senapati‡*

‡Chemical

Sciences Division & §Surface Physics and Material Science Division, Saha

Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Kolkata-700064, INDIA

Corresponding Author *[email protected] Ph# +91-9433139141, Fax: +91-33-2337-4637

ABSTRACT: Polyvinylpyrrolidone (PVP)-based Silver nanoprisms (AgNPrs) show an initial stacking geometry due to their low zeta potential and electrostatic interaction

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between face-to-face energetically stable {111} surface-bound pyrrolidone groups through Na+ ion induced cation- interaction. Congested interplanner space between AgNPrs allows As(III) to react differentially with silver atoms from facial {111} and peripheral {110} facets to results smaller stacking and finally to nanoseeds. Above this critical concentration of As(III), PVP leached-out from nanoparticles to form nanoseed engulfed emulsions and inducing controlled aggregation. This entire morphological transition has been decoded by recording their plasmon and SERS tuning and confirmed by TEM study. Strong affinity and selectivity of As(III) towards Ag atom (verified and estimated by HF/3-21g* level of DFT calculation) coupled with low-cost colorimetry provide us a versatile assay with potential application in the environmental protection drive.

KEYWORDS: Colorimetry, Plasmon, Stacking-Destacking, Tuning-Detuning, SERS, Cation- interaction

1. INTRODUCTION


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Arsenic (As) is one of the most profuse toxic elements on Earth's crust.1-2 Major arsenic species found in environmental samples are either inorganic salts [arsenite: As(III) and arsenate: As(V)] or stable organic compounds [e.g., dithioarsenate (DTA), dimethylarsinic acid (DMA), and monomethylarsinic acid (MMA)].3-5 Sources of As contamination is many fold which include sulfide ores;6 natural phenomena like weathering, biological activity, volcanic eruption; together with anthropogenic inputs and spreads through rain and dry fallout.4-7 According to the 2009 European Food Safety Authority (EFSA), Parma, Italy8 and World Health Organization (WHO), inorganic arsenic is acutely toxic compared to organic arsenic compounds9 and causes skin lesions, peripheral neuropathy, diabetes, cardiovascular diseases, and cancer of the lung, liver, urinary bladder in addition to skin. As a result of this, the detection, quantification and removal of inorganic arsenic from the drinking water gained lot more momentum compared to organic arsenic. The EFSA and WHO fixed the maximum tolerable concentration of arsenic as 15µg/kg of bodyweight and 10ppb in drinking water (old limit of arsenic contamination: 50ppb10). Global hot spots with high arsenic risk include South of Asia (Bangladesh, Mongolia,


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and Malaysia), South of South America (Chili and Argentina) and Western North America. A large population of South and Southeast Asia are seriously exposed to potentially dangerous levels of carcinogenic arsenic via their drinking water every day.11 By knowing its high toxicity, arsenic free drinking water is in high demand for the modern healthier lifestyle. Sufficient reports are also available which explore the positive side of concentration dependent arsenic contaminations

12-13

and doping.14 Hence, the quantification of

arsenic is very important both for blissful effect as well as to tackle translational health hazards and indirectly helping us to grow the subject of heavy metal chemistry.15 Literature is rich with reports for the detection of arsenic either by laboratory-based analytical procedures10,

16-27

or by using noble metal nanomaterials28-34 but the

mechanistic revelation of selective detection of arsenic4,

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not only seldom but also

superficial in nature. Based on their wealth of optical properties (absorption, emission and scattering) the noble-metal nanoparticles find high throughput applications in different advanced fields36-40 which include sensing, diagnostics, therapeutics, optoelectronics, catalysis, alternate energy, etc. Surface plasmon resonance (SPR)41-48


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bands of noble-metal nanoparticles are typically located in the vis-NIR region and are strongly dependent on nanoparticle size, shape, composition, crystallinity, interparticle spacing, and local dielectric environment.42, 45-49 Out of several noble-metal NPs, silver nanoprism (AgNPr) is one of the most promising candidates for multicolor diagnostic labeling purposes because of their easy tunability of the broad-band in-plane dipole SPR simply by adjusting the aspect ratios (ARs) =

𝐿 𝑇

where L = side length of the prism

and T = thickness of the prism and by inducing their 2D & 3D assemblies.50 Yu et al.51 and others52 found that silver nanoplates are sensitive to several inorganic anions and results SPR band tuning between 650 and 450 nm by adjusting the added anions concentration. Though Yu et al.51 explained this as the effect of surface electron injection, Xu et al.53 cites the color change of AgNPr as the sculpting effect of chloride ion which transforms the triangular shape into circular disk-like shape by surface etching. Recently, two groups, Xu et al.53 and Tsai et al.,44 independently found the threshold concentrations for Cl-, Br- and I- as ~3×10-4, ~1× 10-6 and ~1.5×10-6 M respectively compared to ~4×10-4 M of SCN- to initiate the same sculpting process. Like


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surface Plasmon behaviors, noble metal nanomaterials also show interesting inelastic scattering properties and can be verified easily and cost effectively by recording their surface enhanced Raman scattering (SERS) spectra. Like SPR, SERS also show size and shape dependent (of the nanoparticle) enhancement of Raman spectra for the Raman active molecule when they are adsorbed onto a nanosurface due to the change in active surface area (by increasing their individual size or by generating controlled aggregation) to be available for molecular adsorption or a variation in the extent of field enhancement (localized enhanced field: Es) at the nanosurface. Although the literature is rich where SERS has been used to measure the size (Ref.) of the active nano surface or to find out the surface curvature to estimate their relative capability of field enhancement by lightning rod effect (Ref.), a direct SERS method to trace their morphological change during their interaction with specific chemical at different concentration is seldom and attracts our attention to use it for ultrasensitive and highly specific detection. Due to their sharp features (compared to SPR) and strong intensity, SERS can also be used as an efficient sensing tool to understand the morphological transition of nanomaterials during their selective interaction with an analyte but


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obviously not a direct spectroscopic method and depends heavily on the ability of the dye to adsorb on the specific nanosurface. Out of several nanomaterials-based technologies32, 54-57 the colorimetric technique5 for selective and sensitive detection is of most useful for its large scale application associated with its low cost implementation. Most of the nanomaterials-induced colorimetric techniques are based on the formation of controlled aggregation by exploiting their surface bound ligand chemistry in presence of heavy metal ions which tune their SPR to show different colors. They bear a common plasmon feature of shifting to higher wavelength associated with reduction in intensity and broadening. Moreover, in this strategy we don't induce any morphological change of the individual nanomaterials rather simply changes their interparticle relative distance to induce plasmon coupling. Getting inspired from these facts we have tried here to induce morphological change simply by adding increasing concentration of As (III or V) in AgNPr solution which results the tuning of plasmon band as a combined effect of breaking down the AgNPr overlay stacking by arsenic ion intercalation, leaching of disintegrated AgNPrs, removal of PVP surfactant from AgNPr surface to form size variable emulsions and thereby inducing emulsion incorporated controlled aggregations


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or even complete dissolution of AgNPr to form unpredicted composites to lose plasmonic characteristics. Here the characteristics of the polymeric surfactant (functional group, molecular weight, etc) are crucial to induce both overlay stacking as well as emulsion incorporated aggregation. This concentration dependent morphological transition is only sensitive for arsenite [As(III)], leaving apart all other common alkali, alkaline and toxic transition metal ions including arsenate [As(V)] and has been confirmed by TEM. A plausible explanation of the color coded sensing of As(III) has been presented by analyzing their tunable surface plasmon and surface enhanced Raman spectra. This low cost, easy-to-perform and error-free As(III) detection technique will provide us a versatile tool to find enormous application in the field measurements.5

2. EXPERIMENTAL SECTION 2.1. Reagents Silver nitrate (AgNO3, >99%), trisodium citrate (TSC, Na3C6H5O7, ≥99%), sodium borohydride (NaBH4, >98%), PVP (Mw = 10,000g mol−1), hydrogen peroxide (H2O2, 30 wt %), sodium meta-arsenite (NaAsO2, ≥90%), sodium arsenate dibasic hepta hydrate (NaAsO3, ≥98%)


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were purchased from Sigma-Aldrich. All the chemicals were of analytical grade, and used without further purification.

2.2. Synthesis of silver nanoprism The initial triangular silver nanoprisms were synthesized by following the Mirkin’s method58 with slight modification. Briefly, 25mL of Milli-Q water was taken in a 50mL beaker and then 250µL of 10-2M AgNO3 was added drop by drop in it. To this diluted AgNO3 solution we then added 1.5mL of 30mM TSC, 1.5mL of 0.24mM PVP (MW ~10,000) and 60µL of 30 wt % H2O2 successively. Between each addition, 30s time lag was given for proper mixing of reagents. To this final mixture, 250µL of 10-1M NaBH4 was rapidly injected and the solution turns into yellow color initially which becomes colorless after some time. The solution was stirred for additional 30min; the color was darkened to a deep yellow which indicates the formation of small Ag nanoparticles. Over the next several seconds, the color of the solution changes from yellow to blue in a stepwise pattern.

2.3. Instrumentations 2.3.1. Transmission electron microscopy (TEM) measurements were done by using a FEI, Tecnai G2F30, S-Twin microscope operating at 300kV. We have used simple but modified technique as described in our previous reports59, 60 for clean monolayer sample preparation on carbon film 300 mesh copper grid (CF300-CU) from Electron Microscopy Sciences (EMS). 2.3.2. UV-visible extinction spectra were recorded at room temperature in a Jasco V-650 spectrophotometer by using a quartz cuvette with 1cm optical path. 2.3.2.2 Surface enhanced Raman scattering (SERS) experiments were performed by using a homemade Raman setup. We used a continuous wavelength diode-pumped solid state (DPSS)


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laser from Laser Glow Technology, Canada (LRS-0532-PFM-00300-03) operating at 532nm as excitation light source and a constant 30s as integration time (at fixed excitation power of 3mW by using neutral density (ND) filters throughout the experiment). For efficient focusing and filtering we used an InPhotonics, MA, USA made 532nm fiber optics Raman probe with spectral range 200-3900cm-1 (Stokes) and 7.5mm as working distance for sample excitation and data collection. Used Raman probe consists of two single fibers (105μm excitation fiber and 200μm collection fiber) with filtering and steering micro-optics, N.A. 0.22. Excitation fiber was connected to a fiber port to align the exciting laser light (at 532nm) whereas the collection fiber was connected to a spectrometer to collect and analyze the scattered radiation. Objective alignment was kept perpendicular configuration to the sample to achieve maximum scattering collection. A miniaturized QE65000 scientific-grade spectrometer from Ocean Optics was used as Raman detector with spectral response range 220–3600cm-1. Raman spectrometer is equipped with TE cooled 2048 pixel CCD and interfaced to a computer through an USB port. At the end, the Raman spectrum was collected by using Ocean Optics data acquisition SpectraSuite spectroscopy software. Each experiment was repeated for 5–6 times and the average values are reported in this manuscript.

3. RESULTS and DISCUSSION Exploitation of SPR for selective detection of an analyte is long known in the literature5 where we utilize their surface bound ligand chemistry for specificity61, 62 and gigantic cross section (absorption and scattering) of the metal core for sensitivity.63, 64 In


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this scheme we use the surface bound ligand as fishing net and the metal core as

yardstick to identify and quantify an analyte respectively without observing any morphological change of the individual nanoparticles. In another approach of colorimetric detection where the metal core of the nanoparticle acts both as fishing net as well as yardstick and in presence of analyte (ionic or molecular) the unstable (and hence reactive) exposed surface atoms directly react with the analyte to create a permanent morphological change of the nanomaterial. Since the change of color in this approach is due to the analyte induced morphological change of nanoparticles, it does not encourage of forming aggregate or in other words the formed morphological intermediates are very stable to explore their reaction mechanism. In our present study we have adopted the second approach of selective color coded sensing of As(III) by using AgNPr as the nanomaterial and compared the colorimetric result in the same experimental condition for all other common alkali, alkaline and toxic transition metal ions including arsenate As(V). The absorption spectra of the synthesized AgNPrs are shown in Figure 1A where the plasmon band in the range 650-750nm corresponds


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to in-plane dipole mode and the plasmon band near 330nm is due the out-of-plane

quadrupole mode of the nanoprism44.

Figure 1. (A) Stacking length dependent plasmon tuning. TEM images to show the effect of Na+ ion concentration induced cation- interaction on average stacking length variation between 198 and 0nm, where (B) at M (molar ratio between NaBH4 and AgNO3) =16, L=198nm and N=26 the obtained 𝑑(400𝜇𝐿) = 6.4Å with (400𝐿) = ―10.5 𝑚𝑉; (C) at M=10, L=136nm and N=18 the obtained 𝑑(250𝜇𝐿) = 5.9Å with (250𝐿 ) = ―14.6𝑚𝑉; (D) at M=7, L=93nm and N=12 the obtained 𝑑(175𝜇𝐿) = 8.2Å with (175 𝐿) = ―17.2𝑚𝑉 and (E) at M=4 they don't form stacking structures rather distributed as individual prisms with (100𝐿) = ―25.4𝑚𝑉.


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The broad peak in the range 440-460nm is attributed to two closely spaced SPR bands, in-plane quadrupole and out-of- plane dipole modes of AgNPrs. Details of the HRTEM study of these nanoprisms are presented in Figure 2 and in our previous report45 which shows both frontal and sideway bright-field TEM images. The selected area electron diffraction (SAED) indicates that the as prepared nanoprisms are single crystals with a {111} lattice plane as the basal plane (inter planner spacing = 2.36Å). The flat surface of the silver nanoprisms are parallel to the {111} plane as also been suggested in the literature65, 66. The SAED pattern also shows another set of faint spots with hexagonal symmetry which has been indexed as 1/3{422} reflections with a lattice spacing of 2.50Å, indicating the presence of a single twinning boundary within the {111} planes. For single crystal fcc metals these spots are forbidden, but they can appear when there are two twin planes parallel to one another. The appearance of the forbidden 1/3{422} reflection is often observed in silver or gold nanostructures in the form of thin plates or films bound by atomically flat top and bottom faces. In previous literature reports45, 65 it’s already been reported that for thin AgNPrs the atomically flat top and bottom faces have [111] facets and the edges have [110] facets.


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Figure 2. Frontal and sideway bright-field and HRTEM of our synthesized nanoprisms with {111} lattice plane as the basal plane and 1/3{422} reflections indicates the presence of a single twinning boundary within the {111} planes.

Measured thickness of these nanoprisms has been reported as ~7nm.45 Both the relative thickness map and the corresponding line profile indicates their two-dimensional growth.45 The AgNPr synthesized by using 250L of 0.1M NaBH4 has been taken throughout this study which shows a bright blue color with absorption maxima at 704nm.


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The corresponding TEM images in Figure 3A and 3B show a highly overlay stacking structure which transforms into isotropically distributed arrangement by the application of ultrasonic wave at 40% power of 53KHz frequency at 270C for 10s. This implies that the binding force between {111} faces of AgNPrs in the overlay stacking arrangement is quite week and easily breakable by applying week ultrasonic force.

Figure 3. (A-H) Morphological change of AgNPr by destacking followed by leaching of nanoprisms and emulsion induced controlled aggregation of silver seeds at increasing


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concentration of As(III). A1 indicates the as synthesized AgNPrs after the application of ultrasonic wave at 40% power of 53KHz frequency at 270C for 10s. Here different TEM images correspond to the effect of concentration dependent As(III) exposure on AgNPr structure where A, B, C, D and E correspond to the As(III) concentration of 0M, 1-2M, 2-4M, 5-7M and 8-40M respectively. Similarly the frames F, G and H correspond to the effect of As(III) at concentrations of 40-80M, 350-750M and  1mM respectively. A range of concentrations means that the morphology does not change throughout this range. Effect of As(III) at various intermediate concentrations has not been shown to avoid the complicacy of the presentation.

Though the literature is affluent50,

67-70

with reports of AgNPr overlay stacking, the

actual reasons are still unknown. In an earlier report of AgNPr synthesis by using Bis(psulfonatophenyl)phenylphosphinedihydrate dipotassium salt (BSPP) as surfactant also shows axial overlay stacking and has been explained by Mirkin69,

70

and others as a

result of - stacking offered by the benzene rings in triphenyl phosphine group of BSPP. The same explanation (- stacking) of axial overlay stacking also hold for PVP-


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mediated AgNPr synthesis due to the presence of short chain -electron moiety in the N-C=O region originating from each pyrrolidone group of surface adsorbed PVP. Pyrrolidone molecules from Parallelly oriented {111} facets of AgNPrs then can results - stacking but in that case the distance between two flat prisms should not be more than 3.2Å (the limit of - interaction is the sum of the van der Waaals (VDW) radius 𝑂 71 On the between atoms involved where 𝑟𝐶𝑉𝐷𝑊 = 1.7Å, 𝑟𝑁 𝑉𝐷𝑊 = 1.55Å 𝑎𝑛𝑑 𝑟𝑉𝐷𝑊 = 1.52Å).

other hand if we consider cation- interaction72 between available cations (Ag+ and Na+; 𝑁𝑎 𝑟𝐴𝑔 𝑉𝐷𝑊 = 1.72Å 𝑎𝑛𝑑 𝑟𝑉𝐷𝑊 = 2.57Å)

and surface adsorbed N-C=O -electron moieties then the

distance between AgNPrs should fall in the range 6.6-8.3Å and the obtained TEM results indicate that cation- as the most probable interaction pathway between AgNPrs to form an axially stacked sandwich geometry similar to ferrocene, Fe(C5H5)2, structure. Geometrically the cation- stacking in case of PVP is much weaker than the corresponding ferrocene type of molecules73 due to less degree of orbital overlap and can easily breakable by the application of week ultrasonic wave as discussed before. Scheme 1 explains the actual cataion- interaction among axially stacked AgNPrs where the ball-and-stick molecular geometry shows the orientation of surface bound


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PVP molecules and intercalated Na+ ions. Atomic model of this cation- interaction among surface adsorbed PVP molecules through intercalated Na+ ions which enforce them to arrange in an axial stacking configuration is shown in Figure S1 (Section S1) for better understanding the orientation of electrostatic interaction.

Scheme 1. Schematic representation of cation- interaction among surface adsorbed PVP molecules through intercalated Na+ ions which enforce them to arrange in an axial stacking configuration. Here all the interacting atoms and molecules are shown in balland-stick model and -electron cloud in violet color.


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Worth to mention that our zeta potential study (Table 1) confirms that these AgNPrs are negatively charged and their zeta potential reduces gradually as we increase the amount of NaBH4 during the synthesis. This –Ve charge induces repulsion among AgNPrs to destabilize their axial stacking and hence the relative contribution between cation- interaction and the extent of electrostatic repulsion governs the distance between two AgNPs and may vary between idealistic 4Å to a maximum of 10Å. According to the literature, the free energies associated with the crystallographic planes of an fcc metal increases in the order: 𝛾{111} < 𝛾{100} < 𝛾{110}66 and explains the axial stacking through {111} facets. Table 1. Variation of zeta potential () and stacking length (L) as we vary the molar ratio (M) between NaBH4 and AgNO3.

𝑀(𝑁𝑎𝐵𝐻4:𝐴𝑔𝑁𝑂3) 4

7

9

16

 in mV

-25.4

-17.2

-14.6

-10.5

L in nm

0

93

136

198


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Since the reduction potential for Na+, Ag+ and NaBH4 are -2.71, 0.8 and -1.24V respectively, NaBH4 can easily reduce Ag+ but not Na+. Besides that the concentration of NaBH4 added during synthesis is more than enough for complete reduction of Ag+ which rules out the possibility of any free silver ion after AgNPr synthesis. As a result of this, increased amount of NaBH4 raises the concentration of Na+ ions not only to enhance the possibility of cation- interaction and thereby stacking length but also to reduce the effective negative charge of AgNPr assemblies. Obtained AgNPrs show gradual approach towards neutral zeta potential with increase in stacking length, L, as we increase the concentration of Na+ originates from NaBH4. We can define the molar ratio (M) of NaBH4 to AgNO3 as: 𝑀 = 𝑉𝑁𝑎𝐵𝐻4 × 𝑆𝑁𝑎𝐵𝐻4:𝑉𝐴𝑔𝑁𝑂3 × 𝑆𝐴𝑔𝑁𝑂3 Here ‘V’ and ‘S’ represent the volume and molar strength respectively. By varying the molar ratio (M) between 16 and 4 we have achieved different stacking length with unchanged edge length of 50±20nm for individual nanoparticles. By knowing the stacking number (N, average number of AgNPr present in each stack) and thickness of individual AgNPrs (T) we can easily calculate the spacing (di, i = amount of NaBH4)


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between AgNPrs in different stakes as 𝑑𝑖 = {𝐿 ― (𝑁 × 𝑇)}/(𝑁 ― 1). The effect of increasing concentration of NaBH4 on stacking length has been clearly presented in Figure 1B-E where the average stacking length changes between 198nm to 0nm. Though the concentration of Na+ is crucial in making of stacking patterns of AgNPrs, detection and quantification of sodium ions as stacking intercalators is not easy by recording their Energy Dispersed X-Ray (EDX) spectra.74, 75 Detailed reason behind this has been explained in the ESI (Section S2, Figure S2) section. As reported in the early literature76, 77 that As(III) can form a sparingly soluble stable complex with atomic or ionic silver as Ag3AsO3 complex, the chemical affinity between As(III) and atomic silver (Ag0) on AgNPr surface can easily overcome the electrostatic repulsion between AsO2- and the negatively charged NPrs to induce the surface leaching. Upon the addition of increasing concentration of arsenite (AsO2-) the original blue color gradually changes to purple at 1-2µM (75-150ppb), burgundy at 2-4µM, pastel orange at 6-7µM, golden yellow at 8-9µM, yellow at 10-40µM, light yellow at 4080µM and repeated these colors as pastel orange at 90-100µM, brown at 110-150µM, burgundy at 150-200µM, blue violet at 200-350µM, lavender at 350-500µM, deep blue


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at 500-750µM and finally light blue at 750µM-2mM of As(III) (original color of the AgNPr). Corresponding color coding and UV-Vis spectra of each morphologically modified AgNPr is shown in Figure 4A and 4B respectively. To understand the plasmonic change at lower (in the µM concentration) and higher (above 10 µM concentration) concentration of As(III) we have plotted their UV-Vis spectra in two different figures as Figure 4B1 and 4B2 respectively. Both Figure 3 (3A-3E) and Figure 4B1 shows the ability of our assay to detect As(III) in the µM level concentration.


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Figure 4. (A) Concentration dependent color coded sensing of As(III) between the concentration range of 10-6M to 10-3M, (B) Tuning of surface plasmon resonance as a result of morphological change of AgNPr at different concentration of As(III) between 106M

to 10-3M where (B1) shows the variation of plasmon band at different lower

concentration of As(III) in the range of 0.0-10.0µM [0.0µM (blank): Black trace (max=704nm), 1.0-2.0µM: Blue trace, 2.0-4.0µM: Orange trace, 5.0-7.0µM: Red-Violet trace, 8.0-10.0µM: Blue Violet trace] and (B2) at different higher concentration of As(III). The Plasmon band and hence the color of the nanomaterials change in a distinct manner where a specific color remain unchanged in a broader range of growing concentrations as 10.0-80.0µM: Yellow, 90.0-100.0µM: Orange, 110.0-200.0µM: Dark Red, 250.0-500.0µM: Purple, 750.0µM-2mM: Different shades of blue, 3mM-10mM: Faded Blue and above 10mM the color becomes faint blue to gray or almost colorless.

Consequent step-wise morphological change of the nanoprism which shows a distinct color from the previous morphology52 due to the interaction of As(III) with the metallic


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core of the nanoprism, we have clearly represented this phenomena by recording the TEM image for each intermediates and has been portrayed details in Figure 3A-H. Electronic configuration of As is [Ar] 3d104s24p3 and the corresponding As(III) is [Ar] 3d104s2 and As(V) is [Ar] 3d10. Due to the full filled electronic shell structure, As(V) is more stable and inert compared to As(III) having two valance electron to support its reactivity. Moreover due to the bulkier size of arsenate (𝐴𝑠𝑂3― ) compared to the arsenite (𝐴𝑠𝑂2― ), accessibility of inter planer space (6-8Å) in AgNPr stacking by As(V) is less probable than As(III). Along with these two factors, by considering the formation of stable silver arsenite (Ag3AsO3) and silver arsenate (Ag3AsO4) compounds we have drawn them in Gauss View 5 and performed their geometry optimization by using HF/321g* level of theory. The free energy calculation for the reaction: Ag3AsO3 + ½O2→ Ag3AsO4 was found to be -118.5 kcal/mol suggesting that As(III) is more reactive than As(V) for leaching Ag atoms from AgNPr surface. Details of the theoretical calculation have been included in the ESI sectionS3. This relative chemical activity and steric hindrance caused by of As(III) and As(V) will be discussed in the next section as a factor to explain the selectivity and sensitivity of As(III) for color coded sensing. Due to


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the NaBH4 reduction procedure to synthesize AgNPrs which results a nanoparticle solution with pH 8.4 and the formation of stable and sparingly soluble silver arsenite (Ag3AsO3) and silver arsenate (Ag3AsO4) compounds at higher pH, organized assembly of AgNPrs can react instantly with arsenic ions in the intrinsic basic medium. Moreover, due to high pH of the AgNPr solution, it also diminishes the possibility of silver leaching through oxygen reduction reaction (ORR: O2(g) + 4H+(l) + 4Ag(s)  2H2O + 4Ag+).78 As we increase the concentration of As(III), due to the difference in stability between flat surface bound [111] facets and edge bound [110] facets (free energies increases in the order: 𝛾{111} < 𝛾{100} < 𝛾{110}66) the number of pyrrolidone bound silver atoms from edges will be released more compared to flat surfaces. This will result the formation of smaller stackings due to the increase of surface charge originating from pyrrolidone molecules as we observed as an effect of As(III) addition between 1.0-6.0µM. A gradual reduction in the dimension of AgNPr as a result of increasing addition of As(III) (by comparing the dimension between B and E of Figure 2) confirms the removal of pyrrolidone bound silver atoms from [110] facets. Appearance of different surface Plasmon at different lower concentration (between 1.0-6.0µM) of As(III) has been depicted in Figure 4B1


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which matches well with the obtained surface plasmons (Figure 1A) from different stacking lengths synthesized by using different amount of NaBH4. Hence at lower concentration of As(III), appearance of different color is due to the transformation of longer AgNPr stacking into smaller piles. At the range of 6.0-7.0µM of As(III), the stacking of AgNPrs disintegrated completely along with partial leaching of the tips as well as the surface of AgNPrs to produce isotropically distributed silver nanodiscs (average diameter between 15-25nm) with surface plasmon band appears near 450nm (green colored spectra in Figure 4B1) and the corresponding TEM is shown in Figure 1E. If we increase the As(III) concentration further in the range of 8.0-80µM, silver nanodiscs are leached further to generate smaller nanoparticle (silver seeds) with size range 5-7nm which gives different shades of yellow (max=410-430nm) as we mentioned before and has been represented in Figure 3E. Similar structural changes have also been observed as an effect of KSCN addition to light blue colored AgNPr solution reported by Tsai et al.44 Further increment of As(III) concentration (between 90µM2mM), the spontaneous reaction between silver seeds with arsenic removes PVP from


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nanosurface to initiate the formation of PVP-based controlled emulsions with average diameter between 50-200nm which starts incorporating silver seeds and induces controlled aggregation (smaller to larger) with a plasmon shifting between pastel orange to light blue (90-100µM: max=460-470nm, 110-200µM: max=550-560nm, 250-500µM: max=600-610nm and 750µM-2mM: max=680-720nm) and the corresponding TEM images are shown in Figure 3F and 3G. If we increase the concentration of As(III) beyond 2mM, reaction between arsenic and silver is so vicious that it ultimately converts all nanomaterials into silver arsenate (Ag3AsO3) and forms unpredicted composites to loose plasmonic characteristics of silver nanoparticles which loses its blue color as shown in Figure 3H to shift the plasmon band much beyond 704nm and ultimately disappears. Though the generated Ag3AsO3 is yellow, black colored aggregated silver material can easily mask the yellow color precipitation originating from Ag3AsO3. To find out the selectivity of As(III) for the morphological change of AgNPr which results color coded sensing of arsenic contamination, we have repeated the same set of experiments (absorbance, TEM and colorimetry) with several other metal salt which includes common alkali, alkaline and toxic transition metal ions including arsenate As(V)


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and the colorimetric result has been depicted in Figure 5A. To observe a sharp contrast in colorimetry and to avoid any unwanted interference (frommorphological change52) from halogens we have used 30µM concentration for all non-halide metal salts. Except some minor contrast change, colorimetry can selectively and sensitively detect As(III). To confirm that the color change is not due to the pH change of the solution after the addition of the As(III) salt,we have chacked the pH of the blank AgNPr solution and the same solution after the adddition of the As(III) salt. We observed that the pH of all the solution was increasing slightly than that of the blank upon the gradual addition of As(III). We have performed a control experiment by adding equal concentration of NaOH instead of the As(III) salt by considering 1M NaAsO2 to generate 1M NaOH after complete dissolution. The pH in both case is listed in Table S2 in ESI section S4. No color change (Figure 5B) was observed for pH change due to the addition of NaOH which proves that the role of pH for color tuning is negligible.


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Figure 5. Colorimetric change of AgNPr solution upon addition of (A) 30M (left to right) Na+, K+,Ca2+, Mg2+,Sr2+, Cd2+, As3+, Fe3+,Hg2+, Fe2+, Pb2+,Zn2+and As5+metal ions ; (B) NaOH in the concentration range 10µM-1mM; (C) Different concentration of arsenic prone water sample into the AgNPr colorimetric assay (D) 0.1µM NaAsO2- dissolved in Milli-Q, tap water and drain water.

Morphological change of AgNPr upon addition of different concentration of As(III) which result the appearance of different color is also been traced by recording their SERS spectra in presence of crystal violet (CV) as Raman tagging molecule. We have


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taken 5µL of 10-4 M CV and then added to195 µL of the morphology modified colortuned AgNPr solutions which we have obtained by adding same amount of differentially diluted NaAsO2 into same stock solution of AgNPr to avoid any dilution factor. Obtained SERS spectra from different morphology modified AgNPr are depicted in Figure 6 in a relative intensity scale which includes the SERS signal as a result of As(III) addition in the range of 0-5mM. It is clear from Figure 6 that there is a gradual increment of the SERS signal intensity as we increase the As(III) concentration from 0M to 8µM. It is also clear from the TEM results (Figure 3) that the gradual increment of As(III) up to 8µM results the initial destacking of AgNPrs and finally disintegrate each nanoprisms into smaller silver nanoparticles to increase the effective surface area and results a gradual increment of SERS signal for the formation of higher number of hotspots. As we increase the concentration of As(III), color of the resulted nanoparticle solution changes from golden yellow at 8-9µM to yellow at 10-40µM and light yellow at 40-80µM As(III) due to the formation of smaller sized silver nano particles (seeds) and we observed a slight increment of SERS signal. Further addition of As(III) beyond 80µM, SERS signal gradually reduces till 2mM concentration and beyond 2mM concentration of As(III) the


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resultant materials doesn’t show any detectable Raman signal. Details about the mechanism of SERS and the mathematical formulation of the Raman enhancement factor, G, has been described in details elsewhere.79 G can be derived by the following equation: 𝐼𝑆𝐸𝑅𝑆 𝐺=

𝐼𝑁𝑅𝑆

𝑁𝑠𝑢𝑟𝑓 𝑁𝑣𝑜𝑙

where 𝐼𝑆𝐸𝑅𝑆 is the surface enhanced Raman intensity, 𝑁𝑠𝑢𝑟𝑓 is the number of molecules bound to the nanometallic substrate in the excitation volume, 𝐼𝑁𝑅𝑆 is the normal Raman intensity and 𝑁𝑣𝑜𝑙 is the number of molecules in the excitation volume. In our estimation of enhancement factor we have considered the excitation zone as a cube with dimension of 100m or a volume of 10-6cc. By considering the final concentration of CV in SERS experiment and normal Raman experiment as 2.510-6M and 1M respectively, number of molecules adsorbed on the nanosurface and in bulk in the excitation volume has been estimated as 15108 and 61014 molecules respectively. By comparing the Raman peak intensity at 1380cm-1 the obtained enhancement factor (G) in absence and in presence of 8M (close to maximum SERS signal) of As(III) was calculated as 3106 and 8106 respectively. Depending on the increasing concentration of added As(III), the extent of released PVP also increases and allows the system to form different diameter polymeric emulsions to incorporate smaller silver nanoparticle formed in the concentration range of 10-80µM of As(III). Due to bigger and


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thicker PVP layer, the accessibility of crystal violet to the core silver nanoparticles decreases gradually as we are forming different morphologically modified AgNPrs at higher concentration of As(III). This gradual increment of inaccessibility of CV to nanosurface incorporated inside emulsion results gradual reduction in SERS signal as observed in the concentration range between 80µM-2mM. Since the emulsion incorporated nanoparticles looses their plasmonic properties beyond 2mM As(III) concentration, the observed SERS signal at 3mM disappears as depicted in Figure 6. Assignments of different Surface Enhanced Raman line originating from CV have been reported and described elsewhere.80

Figure 6. (Left) SERS based assay to monitor the morphological modification of AgNPr on addition of different concentration NaAsO2 in the concentration range 0-8M and the


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corresponding colorimetric response has been shown by the adjacent anti clockwise arrow; (Right) SERS response in the concentration range 80M-3mM and the corresponding colorimetric response by the adjacent anticlockwise arrow.

To find the applicability of our proposed AgNPr colorimeric-assay we have performed our investigation on real samples. For high specificity sensing and quantification of As(III) from real samples we have collected water from an arsenic prone zone of West Bengal (Duttapukur, North 24 Paraganas),81 India which has been used years as drinking water by the local people. We have taken 1mL of our synthesized blue color AgNPr stock solution in each of six different centrifuge tubes (assay tubes) and then added different volume of arsenic contaminated water sample (filtrated by 0.2m syringe filter) in the range of 0-1mL. Assay tubes containing arsenic contaminated water sample less than 50L do not show any color change while other assay tubes having contaminated water volume more than 50L show usual color change as we have observed in our ideal highly specific colorimetric assay sensing of As(III). Colorimetric transition observed in AgNPr assay upon addition of different amount of arsenic prone


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water sample is shown in Figure 5C. The detection and quantification of As(III) from real sample is not very straight forward and may associate with other problems. One of such prominent problems comes from the considerable presence of additional halides (Cl-, Br- and I-) in the environmental water samples. Morphological effect of halides on AgNPr has been reported earlier52 and discussed in previous section. So the observed color change may originates both from halide ions and As(III). To confirm the origin of color change, we have titrated the water sample against 10-2M AgNO3 solution using KCrO4 as the indicator. From the end point we have calculated the amount of precipitatable salt as ~2.1mM. From the literature52 it is known that the color change of AgNPr is observable only when the concentration of halide ion present in the water sample is 20mM. Since the precipitatable salt concentration is only 2.1mM in our studied sample which is 10-times less than the threshold halide concentration for color change, rules out the presence of any substantial amount of halide which can independently cause color change. Hence, the observed color change is solely originating from As(III) present in the water sample and the threshold concentration for As(III) detection in the micromolar range (1-2M) shows its hypersensitivity. Along with arsenic contaminated


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real water sample we have applied our assay on different sourced water samples collected from Milli-Q filtration system (Synergy, UV), Tap water and drain water from Kolkata region. After initial filtration by 0.2m syringe filter we have infused them separately with NaAsO2 to reach the desired 100M concentration of As(III). Once we add 100L of Milli-Q water as control and As(III) infused water as test samples separately in a set of two 900L blue color AgNPr solution, all of them show desired golden yellow color for the infused samples and no color change for the blank which proves zero interference from other components of water. Since WHO reports that the average halide (mainly Cl-) concentration in water is only 4M (much lower than the interference level) in drinking water, our AgNPr-based assay can successfully be used for As(III) sensing in drinking water without much interference from halide ions.

4. CONCLUSION AgNPr-based assay has been designed and experimental procedure has been elaborated to achieve the wide range morphologically modified surface Plasmon tuningdetuning for accurately color coded detection and sensing of As(III) over other alkali,


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alkaline, and transition metals. Wide range morphological modification of AgNPr is measured and confirmed by TEM measurement and a plausible explanation has been provided to understand the mechanism of selective interaction of As(III) with axially stacked AgNPrs based on their zetapotential, plasmon tuning-detuning and surface enhanced Raman spectroscopy along with DFT calculation. Plasmon tuning-detuning positively not only confirms the presence of As(III) but also can detect the As(III) concentration up to a limit of 75ppb which is close to the old EPA limit for As(III) content in drinking water. This is probably the first report where we have explored the critical role of PVP for wide range morphological modification followed by emulsion induced aggregation to effectively tune the surface plasmon for color coded sensing of heavy metals. Our cross check with As(III) containing real sample from arsenic prone area of West Bengal, India and As(III) infused real water sample from different sources proves the potential applicability of this assay as colorimetric sensor for field application towards environmental protection drive.


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ASSOCIATED CONTENT

Details about EDX abundance, theoretical calculation and controlled pH regulation have been described in the Electronic Supporting Information (ESI) section.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Present Addresses †Department

of Chemistry & Center for Nanotechnology, Indian Institute of Technology

Guwahati, Guwahati-781039, Assam, India, E-mail: [email protected]

ACKNOWLEDGMENT

This work was financially supported by BARD project (PIC No. 12-R&D-SIN-5.040103), DAE, Govt. of India. DS would like to extend his special thanks to Mr. Abhishek Sau, Chemical Sciences Division, SINP for providing Arsenic contaminated drinking


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water and Prof. Krishnakumar S. R. Menon and Mr. Arunava Kar from Surface Physics and Material Science Division, SINP for useful discussions.

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