Spontaneous Formation of Cold-Welded Plasmonic Nanoassemblies

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Spontaneous Formation of Cold-Welded Plasmonic Nanoassemblies with Refracted Shapes for Intense Raman Scattering Andrea Mariño-López, María Blanco-Formoso, Leonardo Negri Furini, Ana SousaCastillo, Ecem Tiryaki, Moisés Pérez-Lorenzo, Martín Testa Anta, Veronica Salgueiriño, Nicholas A. Kotov, Ramon A. Alvarez-Puebla, and Miguel A. A. Correa-Duarte Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00234 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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Spontaneous Formation of Cold-Welded Plasmonic Nanoassemblies with Refracted Shapes for Intense Raman Scattering Andrea Mariño-López,§ María Blanco-Formoso,§ Leonardo N. Furini,§ Ana Sousa-Castillo,§ Ecem Tiryaki,§ Moisés Pérez-Lorenzo,§ Martín Testa-Anta,‡ Veronica Salgueiriño,‡ Nicholas A. Kotov, †,* Ramon A. Alvarez-Puebla,⊥,* Miguel A. Correa-Duarte§,*

§Department

of Physical Chemistry, Biomedical Research Center, Southern Galicia Institute of

Health Research and Biomedical Research Networking Center for Mental Health, Universidade de Vigo, 36310 Vigo, Spain. ‡Department

†Department

of Applied Physics, Universidade de Vigo, 36310 Vigo,

of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, United States.

⊥Department

of Physical Chemistry and EMaS, Universitat Rovira i Virgili, 43007 Tarragona,

Spain and ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain.

ACS Paragon Plus Environment

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KEYWORDS. Cold-welding; plasmonics; thin films; SERS, Raman scattering, self-assembly, nanosheets, spontaneous organization, ultrasensitive analysis

ABSTRACT. Nanostructures with concave shapes made from continuous segment of plasmonic metals are known to dramatically enhance Raman scattering. Their synthesis in solutions is hindered, however, by their thermodynamic instability due to large surface area and high curvature of refracted geometries with nanoscale dimensions. Herein we show that nanostructures with concave geometries can spontaneously form via self-organization of gold nanoparticles (NPs) at air-water interface. The weakly bound surface ligands on the particle surface make possible their spontaneous accumulation and self-assembly at air-water interface forming monoparticulate films. Upon heating to 80 0C, the NPs further assemble into concave nanostructures where NPs are coldwelded to each other. Furthermore, the nanoassemblies effectively adsorb molecular analytes during the migration from the bulk solution to the surface where they can be probed by laser spectroscopies. We demonstrate that these films with local concentration of analytes increased by orders of magnitude and favorable plasmonic shapes can be exploited for the surface enhanced Raman scattering (SERS) for high-sensitivity analysis of aliphatic molecules.


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INTRODUCTION Plasmonic surfaces with complex shapes are needed for many applications such as (meta)optical devices, 1-2 (bio)sensing,3 and catalysis.4 Plasmonic films can be fabricated by gas-phase methods such as physical vapor deposition or e-beam lithography.5 However, these techniques produce plasmonic substrates in low yield and defects in crystal lattice.6 Therefore, in recent years colloidal approaches, taking advantage of the control in shape and size possible for nanostructures, together with the capability of mass production,7 have been suggested. For example, plasmonic films can be directly fabricated in solution at the interface of two immiscible liquids. Such assembly is driven by the decreased interfacial energy when nanoparticles (NPs) are trapped at the interface.8 Although nanoparticle spontaneous assembly at the interface has been demonstrated with, for example citrate stabilized AuNPs at the water-propylene carbonate interface without any covalent functionalization9 or even by the addition of hydrophobic ions into the nanoparticle sol,10 most of these processes require covalent surface functionalization of the NPs to direct their assembly either by electrostatic,11 van der Waals forces,12 or hydrophobic interactions.

This covalent

functionalization can be also exploited to obtain films at the air-liquid interface by DNAprogrammable crystallization in solution,13 or by controlled evaporation of alkyl-functionalized contact between the NP surface and a molecular species is required, exemplified by two rapidly evolving areas of nanoscale NPs.14 However, long surface ligands stabilizing the NPs, passivate their surface, hindering the applications where close plasmonics, surface enhanced Raman scattering (SERS) and catalysis.15 The formation of complex interconnected refracted nanoscale geometries is greatly beneficial for both of these areas due to the strong enhancement of


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electromagnetic field in concave nanostructures especially when they are formed from a continuous segment of plasmonic metal.16 However, their formation is problematic because of their inherent thermodynamic instability. Minimization of the surface area typical for nanocolloids greatly favors spheroidal NPs with minimal asymmetry and thus, results into the reorganization and collapse of refracted nanostructures into much simpler shapes. Realization of facile synthesis of such curved nanostructures at interfaces would be greatly beneficial for SERS analysis for a large variety of molecules because such localization makes the plasmonic films easily accessible to phonons and analytes alike.17 In fact, previous reports on the SERS characterization of different assembled films at the interface yielded superior intensities to those obtained by the sols.18-20 Also important to note that the highest contribution to SERS intensity is made by the first layer of the NPs encountered by the impingent light,21 which favors the use of thin NP films. Conventional methods to perform SERS22 rely either on the spectral acquisition directly on a colloidal solution containing analyte or the casting of the latter onto a plasmonic film. Even for the new implementations of these approaches with colloidal particles with high surface area and dispersability,23-26 SERS spectra are acquired using a minute amount of the actual molecular analytes, thus, limiting the analytic potential of this technique. Herein, we present a method to obtain plasmonic films with optically homogeneity and high reproduciblity based on the self-assembly of NPs in solution and at the air-water interface (Figure 1).

Synergistic of acquisition of favorable geometries, particle localization, and analyte

concentration is made possible capping surface ligands with low affinity to the plasmonic material.27 Such ligands engender the spontaneous accumulation of NPs at the air-water interface and markedly facilitate subsequent self-assembly of the NPs due to the ligand dynamics at NPwater-air-interface. Furthermore, such can be replaced by other molecules present in the solution


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with their subsequent local concentration in the interfacial films and subsequent increase of the detection limit making possible ultrasensitive detection of biological analytes.

MATERIALS AND METHODS


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Materials: Tetrachloroauric acid (HAuCl4·3H2O), tetrakis(hydroxymethyl) phosphonium chloride solution 80% in water (THPC), sodium hydroxide 97% (NaOH), mercaptobenzoic acid 90% (MBA), and thiram were purchased from Sigma-Aldrich. All chemicals were used without further purification. Milli-Q water (18 MΩ cm-1) was used in all aqueous solutions, and all glassware was cleaned with aqua regia before the experiments. Synthesis of 3 nm gold nanoparticles (AuNPs): Au NPs were synthesized by adding 91 mL of milli-Q water to an Erlenmeyer flask. Then, NaOH (300 µL, 0.2 M), THPC (200 µL, 0.068 M), and HAuCl4·3H2O (400 µL, 0.025 M) were added, in this order, to the reaction flask under vigorous magnetic stirring. After 3 min., the obtained brownish solution was stored in the refrigerator. Preparation of cold-welded NP films: Films can auto assemble in one of two different vessels; either in Eppendorf tubes (2 mL) or in volumetric flasks (10 mL). Briefly, 1.8 or 10 mL of the AuNPs solution were added to the vessel and placed in a sealed reactor at 80 oC. Preparation of physically evaporated gold island films: Gold island films of 9 nm mass thickness were prepared in a Balers BSV 080 glow discharge evaporation unit. The metal films were deposited on a preheated (200 °C) glass microscope slide (7059 Corning), which provides a SERS active substrate with an appropriate size distribution of ellipsoidal metal NPs. During the film deposition, the background pressure was 10−6 Torr, and the deposition rate (0.5 Å s−1) was monitored using an XTC Inficon quartz crystal oscillator.28 Preparation of cast and air-dried gold films: AuNPs were produced by the citrate reduction method. Then 100 L of the resulting solution (15 nm AuNPs) were cast on a glass slide air dried.29 Deposition of self-assembled NP films onto different substrates: AuNP films were deposited from the solution onto different substrates: TEM grids, glass slides, silicon wafers, and cellulose


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membrane filters. For TEM grids, glass slides, or silicon wafers, the substrates were introduced perpendicular to the surface and close to the wall of the vessel. After immersing 1/3 of the desirable substrate into the solution, the end of the slide was adjusted to a 90° angle until it was parallel to the surface with the self-assembled NP film on its upper side. For deposition of the NP films onto cellulose membrane filters, an 8 mm diameter circle of membrane was laid over the surface of the solution, in contact with the NP films self-assembled at air-water interface. After a few seconds, the filter containing the NP film was gradually removed. Characterization: UV-Vis-NIR spectra were obtained with Hewlett-Packard HP8453 spectrophotometer on the films deposited on glass. TEM images were obtained using a JEOL JEM1010 or JEM2010 transmission electron microscope operating at an acceleration voltage of 100 or 200 kV, respectively. SEM images were acquired with a JEOL JSM-6700f field-emission scanning electron microscope. AFM was obtained with a Veeco AFM-STM Multimode Nanoscope V. SERS measurements: Characterization involved using 10 mL of the as-prepared AuNPs were contaminated with the appropriate volume of MBA or thiram to reach the desired concentration (10-6 M for MBA and from 10-5 to 10-10 M for thiram). After placing the samples in the sealed reactor at 80 oC, the films containg NPs and their superstructures were removed with glass and characterized. For comparison with other surfaces, a solution of gold citrate colloids was contaminated with MBA at the same concentration. After 24 h, a 100 L aliquot of the solution was cast onto a glass slide and air-died. For the Au island films, substrates were immersed in a solution of MBA 10-6 M for 24 h.


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Spectra were collected with a Renishaw InVia system with a 785 nm laser line and a highresolution grating of 1200 gcm-1. The laser beam was focused at the Au film through a 50x objective, providing a sample area of 1 µm2, with power at the sample of 3.39 mW and acquisition times of 1 second. An area of 10 x 10 µm2 was mapped with a step size of 2 µm.

Figure 1. Reaction pathways for the generation of gold NPs with THPC surface ligands, which generates formaldehyde and hydrogen as reducing agents (a); Formation of the gold film at the airwater interface after 24 h at 80 ºC (b). Accumulation and self-assembly of the NPs at the air-water interface (c). RESULTS AND DISCUSSION Figure 1a shows a schematic pathway to produce the initial Au nanoparticles (AuNPs) with low affinity surface ligands. This synthesis was adapted from Duff et al.30 and relies on the reduction of gold (III) in the presence of tetrakis(hydroxymethyl)phosphonium chloride (THPC) in alkaline media in water. Under these conditions, phosphorus (V) is transformed into phosphorus (III), as tri(hydroxymethyl) phosphine, generating formaldehyde, can reduce gold (III). Further, the


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phosphorus (III) intermediates has enough reducing power to produce hydrogen from water reducing more gold (III) and evolving again to phosphorus (V) in the form of phosphine oxide. This reaction yields small NPs of about 3 nm in diameter (Figure 2, 0h) with a brownish color but without localized surface plasmon resonance. These AuNPs, weakly stabilized with THPC (AuNP@THPC), remain stable in solution at normal conditions. However, when the solution is placed in a closed container (to prevent evaporation) thermostated at 80 ºC, the formation of a NP monolayer at the air-water interface is observed (Figure 1b). In agreement with theoretical treatment of interfacial equilibria by Pieranski,8 particles at the interface are trapped in a surface energy well that can be described as: 𝜋𝑅2

∆𝐸𝐴𝑊 = ― 𝛾𝐴𝑊[𝛾𝐴𝑊 ― (𝛾𝑃𝑊 ― 𝛾𝑃𝐴)]2 < 0

(1)

where R is the particle radius and  is the interfacial energy at the air-water, particle-water and particle-air interfaces. Thus, the minimization of the total Helmholtz free energy (EAW) is the driving force for the translocation of the AuNPs from the bulk to the air-water interface. On the other hand, considering Eq. 2,31 𝐽𝐴𝑑𝑠 = 2𝑛0 𝐷𝑡 𝜋

(2)

where JAds is the particle flux to the interface; n0, the number density of the particles in the bulk dispersion; t, time; and D is the diffusivity of the spherical particles, which, according with the Stokes-Einstein equation: 𝐷 = 𝑘𝑇 6𝜋𝜇𝑎

(3)

where k is the Boltzmann constant; T, the temperature; , the viscosity; and 𝑎, the particle radius, it becomes clear that the accumulation of the particles in the interface is promoted, at a fixed concentration, by the increase in the temperature and their small size (Figure 1c).


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Figure 2. TEM images of the evolution of the film structure at the air-water interface as a function of assembly time. Figures 2 and 3 show TEM images of the films transferred to TEM grids at different times. In contrast with other plasmonic films produced at the air-liquid interfaces, where the particles form discrete unconnected films,32,33-34 Au-THPC NPs self-assemble into dendritic structures even after short times (2 h) of thermal treatment. The balance between repulsive and attractive forces on NPs makes formation of NP chains particularly favorable.35 In addition to that, the low affinity of the THPC ligand for the gold surfaces, accelerate their reconfiguration and self-assembly. After arrival at the interface and formation of a monolayer, it is likely that THPC remains at the particle-


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liquid interface, while abandoning the particle-air interface. Thus, the air-liquid interface generates asymmetric NPs trapped at the interface. This reconfiguration of the surfactant layer originates from the optimization of electrostatic interactions and simplifies the merger of the inorganic cores of the NPs. Note that we do not see the strong evidence of epitaxial registry between the NPs known as oriented attachment mechanism.36-37 As the diffusion barrier for a single metal atom on a metal surface is typically less than 1 eV,38 the thermal activation at 80 ºC is enough to overcome such a low barrier when the NP are assembled closely together, resulting in the formation of larger plasmonic structures with NP-NP boundaries ‘cold welded’ together.39 Similar processes have also been observed for semiconductor NPs.35 The geometrical evolution of the film with time shows that these structures grow into curved complex geometries with concave and convex surfaces, which are metastable, From the perspective of thermodynamics the obvious preference to such asymmetric structures is quite remarkable and can be associated with the asymmetries of the NP surface with low affinity surface ligands.40 The NP merger proceeds only to a certain point (Figure 2, 24 h), with no further changes from this point onwards, due to the decline of NPs in solution and inherent self-limiting mechanisms typical for many NP assemblies.41


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Figure 3. TEM images of the films at different assembly times.

The as-formed films with concave NP assemblies made from a continuous segment of plasmonic metal can easily be transferred to any surface. Figure 4a shows a film transferred to glass for its optical characterization. The self-assembled NP films can also be deposited on cellulose filters or silicon wafers with similar morphological patterns (Figure 5). Evolution of the LSPR peaks is shown in Figure 4b. No plasmon resonance can be identified from the as-prepared 3 nm NPs,


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which is consistent with their small size. However, after a short time of incubation at 80 ºC, an LSPR band can be observed with a maximum at ca 520 nm. As the interfacial self-assembly proceeds, the LSPR band slowly redshifts and increases in intensity reaching a maximum at about 740 nm after 24 h. From this time onwards, consistent with the morphological characterization (Figure 2 and 3), optical properties remain unchanged.

Figure 4. Optical, SEM, and AFM images of an optimized film (24 h at 80 ºC) supported on glass (a). Variation of the LSPR of the film supported on glass at different times of formation (b). SERS spectrum of MBA (c). Intensity of the ring breathing (1079 cm–1, highlighted in yellow in the spectrum) as a function of the formation over time. Red and yellow tabs represent the intensity of other common SERS substrates: cast and air-dried Au (15 nm) colloids on glass and 9 nm evaporated Au films on glass (d). Optical and SERS images (1079 cm–1) of the films of an area of 10×10 mm2; [MBA]= 1×10–6 M (e).


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Figure 5. SEM images of the film after 24 h on different substrates. Localized surface plasmon resonances and SEM images for other conventional substrates used in SERS; cast and air-dried colloidal solutions and gas-phase evaporated gold island films. To characterize the SERS performance of the assemblies of THPC-Au NPs obtained at different times, a well-known molecular probe (i.e., 4-mercaptobenzoic acid, MBA),42 was employed. To fully exploit the spontaneity of the NP migration to and localization at interface, minute concentrations of MBA (1 × 10–6 M) were directly added to the as-prepared NPs. Then the NP films were recovered at different times after incubation at 80 0C. All the films produced the characteristic SERS spectra of MBA, dominated by the ring modes at 1589, 1079, and 522 cm–1, explicitly assigned to CC stretching, ring breathing, and out-of-plane ring deformation, respectively. Monitoring the ring breathing mode over time (Figure 4d), shows an exponential increase in the intensity reaching a plateau after 24 h, which fully agrees with LSPR and TEM


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characterization. These results contrast with those obtained with conventional substrates such as cast gold colloids or evaporated gold island films (Figure 4d and 5). Firstly, intensities are considerably larger for the cold-welded films as compared with the other substrates with enhancement factors reaching 2.9 108 for MBA. This noticeable intensity increment is due to the fact that, while with conventional substrates only a minimal portion of the analyte is sampled (i.e., a small volume of a large sample, usually L or mL), in the cold-welded NP assemblies, the analyte is first adsorbed and concentrated onto the nanoparticles that, later on, migrate to the interface to form the film, which considerably increments its local concentration at the measurement point. Time required for the film formation also ensures the thermodynamic adsorption equilibrium and that the maximum retention of the analyte on the cold-welded film. Second, for the cold-welded films, the standard deviation from point to point is significantly smaller than that in conventional substrates, which is corroborated by the SERS imaging of the different materials (Figure 4e). While the cold-welded films show highly homogeneous intensities from point to point over the entire surface, both cast and evaporated films display great divergence as a consequence or the random organization of their electromagnetic hot-spots.43


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Figure 6. Theoretical (unscaled) and experimental Raman spectra of thiram. SERS spectra of thiram in water contaminated with different concentrations of the analyte after adding the nanoparticles and waiting 24 h for the formation of a stable cold-welded film (left); Correlation of the peak area of the CS stretching with the thiram concentration (right). Both high SERS intensity and intensity homogeneity suggest the cold-welded films can serve as optimal substrates for SERS quantification. To test this property, we selected, as an example, the aliphatic fungicide thiram, which is used extensively in crops and whose chronic toxicity seriously affects the thyroid and liver. As an aliphatic molecule, thiram is, in principle, not suitable for SERS analysis because of its small SERS cross-section.44-45 Figure 6 shows the experimental and theoretically calculated Raman spectra of thiram (DFT B3LYP/6-311+G(d,p)).46 Both DFT and experimental Raman spectra correlate well in the band position allowing for an accurate assignment of the vibrational modes. The Raman spectra is dominated by the CS stretching bands (394, and 974 cm–1), the SS stretching mode (537 cm–1), and the SCS and CNC scissoring (316 and 441 cm–1, respectively). In the SERS spectra of thiram (Figure 4), obtained in the same way


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as those for MBA, the dominant vibrational modes are SS and CS stretching modes (555 and 1369 cm–1, respectfully) with minor contributions of the CNC scissoring (441 cm–1), CS asymmetric stretching (930 cm–1), and CN stretching (1138 cm–1). The vibrational pattern is clearly recognized for concentrations less than nM; and, by correlating the peak area of the CS stretching mode (1369 cm–1) with concentration, a linear relation can be stablished, thus demonstrating the applicability of the films for the ultra-quantitative determination of aliphatic molecules in water. This concentration is competitive and even better with those provided by the standard techniques (chromatography, polarography, voltamperometry of luminescence) which provide detection limits ranging from the M to the nM regimes.47 In summary, we demonstrate the possibility of forming plasmonic nanostructures with metastabled concave geometries through self-assembly of small NPs with weakly-bonded THPC surface ligands. The NPs migrate from the bulk solution to the interface where they become trapped at the liquid-air interface. The reconfiguration of the THPC layer facilitates the assembly and mergers or the NPs into nanostructures with refracted geometries. The characterization of the optical properties of the resulting films show very homogeneous films with a high density of hotspots. Both properties, the accumulation of particles at the surface and the generation of dense and homogeneous hot spots, are exploited for ultrasensitive quantification of aliphatic molecules in water. Particles in solution adsorb the analyte and concentrate it at the surface, increasing its local concentration by orders of magnitude. This is demonstrated by the quantitative analysis of an aliphatic molecule known to have very low cross-section for SERS. AUTHOR INFORMATION Corresponding Authors * N.A.K.: [email protected]


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* R.A.A.-P.: [email protected] * M.A.C.-D.: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was funded by MINECO (CTQ2017-88648-R, CTM2014-58481-R, CTM201784050-R), Xunta de Galicia (Acc. 2016-2019 and EM2014/035), Generalitat de Cataluña (2017SGR883), URV (2018PFR-URV-B2-02 and 2017EXIT-08), the European Union (ERDF) and the National Science Foundation (NSF 1463474 and NSF 1566460).

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Spontaneous Formation of Cold-Welded Plasmonic Nanoassemblies

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