Silver nanoflowers with SERS activity and unclonable morphology

Highlights

• Silver nanoflowers with unique morphology via on-surface growth strategy.

• The time resolved growth shows the formation of flowers from the circular contact lines of hemispherical polymer domains.

• High SERS activity of Ag nanoflowers.

• Unclonable morphology coupled with SERS for anticounterfeiting applications.

Abstract

This manuscript presents surface-growth of silver nanoflowers with high surface-enhanced Raman scattering (SERS) activity and unique morphology. The nanoflower morphology is mediated by the seed-mediated growth of silver nanostructures over gold nanoparticles immobilized on hemispherical polymer features. Randomly positioned hemispherical polymer features are self-assembled via surface dewetting of a nanoscopic film of poly (2-vinylpyiridine) (P2VP) film on the polystyrene-grafted substrate. The time-dependent growth study shows that the silver nanoflowers evolve from the circular contact lines separating the P2VP features and underlying substrate. The three-dimensional nature of the underlying feature facilitates growth in a flower like morphology. The silver nanoflowers exhibit high levels of SERS activity allowing for detection of rhodamine 6G at a 100 pM level under 532 nm laser excitation. A feature matching algorithm-based image analysis confirms the unique morphology of every single silver nanoflower. The direct fabrication of silver nanoflowers at random positions with unclonable morphology and high SERS activity is particularly promising for anti-counterfeiting applications.

Introduction

Silver nanoflowers (Ag NFs) are appealing for a range of different applications in sensing, catalysis, and antibacterial coatings [[1], [2], [3]]. The interest in such flower-like nanomaterials emerges from their unique structure. Probing the underlying mechanism leading to such complex structure is scientifically significant to improve our understanding of the nanoscale processes [4,5]. Furthermore, NFs have interesting structure-dependent properties. NFs have large surface area to volume ratio, sharp tips, three-dimensional architecture, and closely spaced components (i.e. petals). In the case of Ag, these structural characteristics are particularly important for the light-matter interactions at the nanoscale [6,7]. Free electrons of silver confined within NFs result in localized surface plasmon resonances. The localization of light near the petals, for example, results in a strong enhancement of Raman scattering. This phenomenon referred to as surface-enhanced Raman scattering (SERS) is important for an increasing number of applications including sensing and anti-counterfeiting [8,9]. The complex morphology of Ag NFs favors the formation of plasmonic hot-spots, where the strength of electromagnetic fields is high [[10], [11], [12]]. Therefore, plasmonic hot-spots result in high SERS activity and enable observation of Raman scattering for very low concentrations of molecules.

Different approaches have been studied to fabricate Ag NFs. Dramatic improvements in material properties can be achieved by adjusting not only the size but also the shape since different crystal planes of nanostructures with such well-defined morphology have different atomic densities and electronic structures [13]. However, since most noble metals adopt a face-centered cubic (fcc) structure, such special architectures need a particular driving force [14]. Therefore, it is still very difficult to prepare branched nanostructures in an easy and controllable manner. Three basic strategies are employed to create these complex nanostructures. In the first approach, the homoepitaxial growth mechanism is activated and dendritic nanocrystals grow around the seed, where surfaces with similar atomic arrangements approach each other, [15]. Usually such mechanisms require high temperatures, e.g. 80–200 °C. In another approach, specific capping agents such as polyvinylpyrrolidone, poly (ethylene glycol), cetyltrimethyl ammonium bromide are used to induce anisotropic growth [[16], [17], [18], [19], [20]]. With this approach, Lu et al. synthesized Ag NFs in colloidal form by chemical reduction of [Ag(NH₃)₂]⁺ in the presence of ascorbic acid, trisodium citrate, and polyvinylpyrrolidone at room temperature [21]. Although this method is successful in the production of branched nanostructures, it is laborious to remove these adsorbed species for later catalytic and sensing applications. In the third approach, polymers [22], DNA molecules [23], aluminum powder [24], anodic aluminum oxide [25] are used as templates to guide the formation of Ag NFs. Wu et al. fabricated Ag NFs using photoresist pillar as the template and reported SERS-based detection limit of 10⁻¹⁰ M for rhodamine 6G with 80% stability after 21 days [12]. In another study, the self-assembly of diphenylalanine peptide was used to obtain Ag NFs on the graphite surface for antibacterial applications [26].

An interesting set of applications in anti-counterfeiting can be enabled by using Ag NFs. An effective approach in the fight against counterfeiting involves fabrication of security labels with distinct features. In addition to their well-defined geometry (e.g. barcodes), these security labels employ additional mechanisms to make their imitation difficult [27]. Light-matter interactions, for example, are intensively explored for these applications. Raman scattering, fluorescence, and light absorption through engineered nanoscale materials are often used for this purpose [[28], [29], [30]]. Raman scattering from molecules is gaining increasing attention for encoding information at a molecular level [31,32]. The spatial and out of plane patterning of nanoscale materials and Raman-active molecules, together with modulating their polarization and intensity have been reported as effective approaches. One challenge associated with such geometrical patterning is the ability of advanced counterfeiters to duplicate these patterns. This issue sparked interest in exploration of systems with random responses in security labels. Commonly referred as physically unclonable functions, the random positioning of materials together with randomized light-matter interactions have recently been demonstrated [33,34]. In this sense, Ag NFs with unique and complex morphology have significant potential as an encoding element. Together with their complex morphology, the ability to directly grow Ag NFs at randomized positions with high SERS activity will enable multi-layered security labels with dual source of randomness.

In the following, we report confined growth of Ag NFs from Au nanoparticle (NP) seeds immobilized over randomly positioned hemi-spherical features. The growth is mediated with a seed selective chemical reducing agent, hydroquinone. Seed NPs are selectively assembled at high densities over randomly positioned hemispherical patterns of poly (2-vinylpiridine) (P2VP). These hemispherical features are fabricated via a self-assembly approach based on solid state dewetting of P2VP on a low surface energy surface consisting of end-grafted polystyrene. The key aspect of the presented growth strategy is that the three-dimensional nature of the hemispherical platform supports growth of silver from edges to the center, as revealed by the time resolved study. The result is the formation of flower like nanostructures confined within three-dimensional volumes defined by the hemispherical features. In addition to their structural characterization, the SERS-activity of Ag NFs is explored. The results show relatively uniform and intense SERS signals through the Ag NFs, enabling picomolar level sensitivity in the detection of the molecule. Despite the similar appearance of Ag NFs to the unaided eye, advanced image analysis techniques show that each NF has a unique morphology. Image matching algorithms can successfully identify a particular Ag NF. The direct fabrication of randomly positioned Ag NFs with unclonable morphology and SERS activity show strong promise for authentication applications.