Flexible Three-Dimensional Anticounterfeiting Plasmonic Security Labels: Utilizing <i>Z</i>‑Axis-Dependent SERS Readouts to Encode Multilayered Molecular Information

Current surface-enhanced Raman scattering (SERS)-based anticounterfeiting strategies primarily encode molecular information in single two-dimensional (2D) planes and under-utilize the three-dimensionality (3D) of plasmonic hot spots. Here, we demonstrate a 3D SERS anticounterfeiting platform, extending “layered security” capabilities from 2D to 3D. We achieve this capability by combining 3D candlestick microstructures with 3D hyperspectral SERS imaging to fully resolve at least three layers of encoded information within the same 2D area along the <i>z</i>-axis, notably using only a single probe molecule. Specific predesigned covert images can only be fully recovered via SERS imaging at predetermined <i>z</i> values. Furthermore, our 3D SERS anticounterfeiting security labels can be fabricated on both rigid and flexible substrates, widening their potential usages to curved product surfaces and banknotes

Transformative Two-Dimensional Array Configurations by Geometrical Shape-Shifting Protein Microstructures

Two-dimensional (2D) geometrical shape-shifting is prevalent in nature, but remains challenging in man-made “smart” materials, which are typically limited to single-direction responses. Here, we fabricate geometrical shape-shifting bovine serum albumin (BSA) microstructures to achieve circle-to-polygon and polygon-to-circle geometrical transformations. In addition, transformative two-dimensional microstructure arrays are demonstrated by the ensemble of these responsive microstructures to confer structure-to-function properties. The design strategy of our geometrical shape-shifting microstructures focuses on embedding precisely positioned rigid skeletal frames within responsive BSA matrices to direct their anisotropic swelling under pH stimulus. This is achieved using layer-by-layer two photon lithography, which is a direct laser writing technique capable of rendering spatial resolution in the sub-micrometer length scale. By controlling the shape, orientation and number of the embedded skeletal frames, we have demonstrated well-defined arc-to-corner and corner-to-arc transformations, which are essential for dynamic circle-to-polygon and polygon-to-circle shape-shifting, respectively. We further fabricate our shape-shifting microstructures in periodic arrays to experimentally demonstrate the first transformative 2D patterned arrays. Such versatile array configuration transformations give rise to structure-to-physical properties, including array porosity and pore shape, which are crucial for the development of on-demand multifunctional “smart” materials, especially in the field of photonics and microfluidics

Transformative Two-Dimensional Array Configurations by Geometrical Shape-Shifting Protein Microstructures

Two-dimensional (2D) geometrical shape-shifting is prevalent in nature, but remains challenging in man-made “smart” materials, which are typically limited to single-direction responses. Here, we fabricate geometrical shape-shifting bovine serum albumin (BSA) microstructures to achieve circle-to-polygon and polygon-to-circle geometrical transformations. In addition, transformative two-dimensional microstructure arrays are demonstrated by the ensemble of these responsive microstructures to confer structure-to-function properties. The design strategy of our geometrical shape-shifting microstructures focuses on embedding precisely positioned rigid skeletal frames within responsive BSA matrices to direct their anisotropic swelling under pH stimulus. This is achieved using layer-by-layer two photon lithography, which is a direct laser writing technique capable of rendering spatial resolution in the sub-micrometer length scale. By controlling the shape, orientation and number of the embedded skeletal frames, we have demonstrated well-defined arc-to-corner and corner-to-arc transformations, which are essential for dynamic circle-to-polygon and polygon-to-circle shape-shifting, respectively. We further fabricate our shape-shifting microstructures in periodic arrays to experimentally demonstrate the first transformative 2D patterned arrays. Such versatile array configuration transformations give rise to structure-to-physical properties, including array porosity and pore shape, which are crucial for the development of on-demand multifunctional “smart” materials, especially in the field of photonics and microfluidics

Formulating an Ideal Protein Photoresist for Fabricating Dynamic Microstructures with High Aspect Ratios and Uniform Responsiveness

The physical properties of aqueous-based stimuli-responsive photoresists are crucial in fabricating microstructures with high structural integrity and uniform responsiveness during two-photon lithography. Here, we quantitatively investigate how various components within bovine serum albumin (BSA) photoresists affect our ability to achieve BSA microstructures with consistent stimuli-responsive properties over areas exceeding 10⁴ μm². We unveil a relationship between BSA concentration and dynamic viscosity, establishing a threshold viscosity to achieve robust BSA microstructures. We also demonstrate the addition of an inert polymer to the photoresist as viscosity enhancer. A set of systematically optimized processing parameters is derived for the construction of dynamic BSA microstructures. The optimized BSA photoresists and processing parameters enable us to extend the two-dimensional (2D) microstructures to three-dimensional (3D) ones, culminating in arrays of micropillars with aspect ratio > 10. Our findings foster the development of liquid stimuli-responsive photoresists to build multifunctional complex 3D geometries for applications such as bioimplantable devices or adaptive photonic systems

Layer-By-Layer Assembly of Ag Nanowires into 3D Woodpile-like Structures to Achieve High Density “Hot Spots” for Surface-Enhanced Raman Scattering

The surface-enhanced Raman scattering (SERS) “hot spots” are highly localized regions of enhanced electromagnetic field within a SERS substrate that dominate the overall SERS intensity. This results in inhomogeneous distribution of SERS intensity in a SERS substrate, thus limiting their application as reproducible and ultrasensitive sensing platforms. Here, we address this challenge by fabricating Ag nanowires into three-dimensional (3D) woodpile-like platforms via layer-by-layer Langmuir–Blodgett assembly. We focus on promoting strong electromagnetic coupling between parallel and vertically stacked Ag nanowire pairs within the woodpile structure to achieve a high density of “hot spots” across the entire 3D SERS substrates. Raman mapping (<i>x</i>–<i>y</i> plane) demonstrates that all of the 3D Ag nanowire arrays exhibit a homogeneous SERS Raman intensity over a large area, whereas their monolayer counterpart experiences >50% of zero and/or an undetectable SERS signal. The SERS enhancement factor increases from 3.1 × 10³ to 2.6 × 10⁴, as the assembled Ag nanowire layer increases from monolayer to three layers, respectively. We attribute the homogeneous SERS signal to the high density of “hot spots” arising from the vertical and lateral gaps within the woodpile layers. The SERS signals plateau off when the number of layers increase from three to five, which can be attributed to limited laser penetration depth. The assembled multilayered silver nanowires demonstrate a larger SERS depth cross section and angle-independent SERS intensity, making such woodpile 3D SERS substrate more reliable and versatile for future sensing applications

Direct Metal Writing and Precise Positioning of Gold Nanoparticles within Microfluidic Channels for SERS Sensing of Gaseous Analytes

We demonstrate a one-step precise direct metal writing of well-defined and densely packed gold nanoparticle (AuNP) patterns with tunable physical and optical properties. We achieve this by using two-photon lithography on a Au precursor comprising poly­(vinylpyrrolidone) (PVP) and ethylene glycol (EG), where EG promotes higher reduction rates of Au­(III) salt via polyol reduction. Hence, clusters of monodisperse AuNP are generated along raster scanning of the laser, forming high-particle-density, well-defined structures. By varying the PVP concentration, we tune the AuNP size from 27.3 to 65.0 nm and the density from 172 to 965 particles/μm², corresponding to a surface roughness of 12.9 to 67.1 nm, which is important for surface-based applications such as surface-enhanced Raman scattering (SERS). We find that the microstructures exhibit an SERS enhancement factor of >10⁵ and demonstrate remote writing of well-defined Au microstructures within a microfluidic channel for the SERS detection of gaseous molecules. We showcase in situ SERS monitoring of gaseous 4-methylbenzenethiol and real-time detection of multiple small gaseous species with no specific affinity to Au. This one-step, laser-induced fabrication of AuNP microstructures ignites a plethora of possibilities to position desired patterns directly onto or within most surfaces for the future creation of multifunctional lab-on-a-chip devices